That was Holton Hill that tapped that ball that Zylstra hauled in. I've heard good things on Zylstra also, but I don't think he's in Thielen's class of athleticism...he's underrated that way. Good hands for sure, but slow. Very well could make the team, but he won't burst on the NFL scene anywhere near what Adam has done.
Apparently, Treadwell has had a great camp, also. I just don't see what could it would do to keep 2 WRs that have very similar skill sets (bigger body, good hands but very slow and can't separate). You need a slot guy like Kendall Wright.