Aplogies in advance for a long post; this topic can generate real passion, so I've used a lot of words in hopes of not being misunderstood ...
Note also that in the below I only claim to speak for myself (!).

My sense is that few who have done a lot of long distance hiking fall into the "purist" camp. As it turned out, I was a pretty straight-forward white-blazer on the AT, just because I rarely saw the point of taking alternate routes on that trail --- in big contrast to the CDT.


Yes.

The PCT, like the AT, is one unambiguous trail that a person can start at one end of and walk to the other end of, being pretty certainly right on the trail for the vast majority of it. NOBO thru's go through the Sierras in a lot of snow, and so of course cannot walk the precise line (how purist gonzo does a person want to be?). There are not a lot of alternate PCT routes ("blue blaze" sort of thing). The one that comes to mind, however, is one that everyone takes, the Eagle Creek alternate in northern Oregon. If taking the more boring official PCT route there is a requirement then I dare say the vast majority of PCT thru-hikers aren't qualified according to self-appointed purists. Up until recently, the forest service had a designated alternate they told people to take in northern WA --- would a person not qualify as thru-hiking if the official route was just closed for years? (In fact, most thru's walked the closed route anyway, but still, some did take the designated alternate).
All that said, I think what makes thru's on the PCT more relaxed about things is that after walking some hundreds or thousands of miles, most folks will have had the pre-trip purist nonsense knocked out of their heads just by personal experience and from all of their social interactions being with other thru-hikers. And a lot of PCT thru's have already hiked the AT, so overall it's a bit of a more mature/experienced trail culture, especially just starting out on the trail.

I think the more normal approach, at least by folks who have actually done any significant walking, is something along the line of "Start at one end of the trail, walk to the other in the same 12-month period, staying on the official trail where this is even reasonably possible and makes sense". I.e., don't hitch hike around parts, don't take easier routes just because they're easier, etc. To be clear, I definitely would not say that there's perfect consensus among thru-hikers on this stuff, but still --- I've not met any triple crown dyed-in-the-wool purists.

This is a bit of a touchy subject because the reality is that there always seem to be some route trade-offs, and sometimes they feel uncomfortable to the thru-hiker, such that in some cases even they are left questioning their own decision between unhappy trade-offs. On the PCT, a common one is fire closures --- walk through illegally (no thanks ...), hitch-hike around and optionally come back later, or walk around? The choices are difficult. On the AT for me and folks around me, it was snow; I did one walk-around where there was butt-deep snow and no one was making it through (I had an early start). On the PCT I hitched around multiple fire closures, but did go back to pick up the missing pieces; this was something I chose to do, and I don't presume to judge those that for whatever reason did not or could not (!). The CDT is just a mass of alternates, one that a recently-defunct group (CDTA) tried to codify into one official route --- but it's a bit more of a "wild west" do-your-own thing approach in reality. Some routes that most thru-hikers walk aren't the so-called CDTA "official" route, and the most commonly used maps (Ley) show a host of alternatives (so-called purple routes, similar in concept to blue blazes).

To get recognition by PCT and AT groups for a thru-hike, the thru-hiker signs a statement of what s/he did, and gets in return a little paper certificate (well, now also a big honking medal for the PCT). You can view these statements online if you're curious, pcta.org and I think aldha (not at all the same organization as aldha-west, go figure). They're less restrictive than the more, typically AT-oriented purist mind set would suggest. Even less restrictive is what a person signs for the triple crown award.

Certainly flip-flopping is not against the rules (unless of course you set it as a personal rule for your own hike --- for example, until the CDT, I had a rule to never slack pack, gave it up a couple of times on the CDT when specific conditions made it just silly).

I think that the strongest sense, the one perhaps with the most consensus, is that a person walks an unbroken line, so that if you hitch into town to resupply, you get back to where you left the trail to continue hiking from there. The idea that a person would do so while literally never missing an inch of the "official trail" is IMO an imaginative exercise by armchair purists.

Now, another question is on what trails is it reasonable to use the term "thru-hiking". When I first heard folks talking about thru-hiking the JMT, my very parochial and initial reaction was "But it's just 300 miles or so". Back in more normal life, it seems now completely normal and reasonable to me to use the term for the JMT or the CT or the LT or a number of other trails measured in hundreds rather than thousands of miles. Really, if I walk a 20-mile trail from one end to the other I could say that I "thru-hiked it". But I would feel a bit silly in doing so! :-)