Some condensation on the inside of a single wall tarp/tent is normal - especially when there is a large temp drop at night.

You don't typically need to waterproof silnylon, although most things made of it do not come seam sealed (meaning water can come through the thread holes where it is sewn). If you wanna seam seal it go buy a tube of mcnett's "silnet" seam sealer, or mix up your own from 1/2 GE clear silicone sealant, and 1/2 white gas, and paint it on the seams with a small brush. However, from your description, it sounds like what you were seeing is condensation from warm moist ground and cold air overtop.

As far as condensation goes, you attempt to minimize it through ventilation, and keep a small towel or hanky to wipe off the inside. (I take about a 1 foot square piece of old cotton t-shirt on all my trips to act as a handkerchief and wiper - some like bandanas, etc.). With anything single wall remember the tighter you are buttoned up with less ventilation, the more condensation you may see.

Note, you might want to tell us what make of tarptent/bugnet and poncho it is if you know. It's very likely someone here will have experience with your particular shelter.