This is a well-meant requirement. I am pretty sure the reasoning behind this is to exclude cotton sweat sox, which can be a source of problems. However, it might have been better if the organizer had described the reasons for avoiding some sox and taking others.

The best all-around hiking sox out there these days tend to be the lighter weight merino wool sox, such as Smartwool. They are quite different from heavy ragg wool sox you might be familiar with. They wick moisture well, dry quickly, are durable and are not especially hot on your feet in hot weather.

If you talk to the leader and show him you have the personal knowlege and experience to select sox that you can demonstrate won't blister your feet or cause sore or hot spots, then I am sure he will agree you can select the sox you know work best for you.

If you really aren't that experienced or knowlegable, but simply think the leader is wrong, talk to him anyway, but don't expect to convince him.