Part of the disconnect here is the gap between "#1" and "among the best #1s." Treadwell is a top-tier #1, and I think our idea of Greenberry, at least, is that he is as well. Assuming the injury doesn't change Treadwell, I think we expect both to not just be drafted (a feat 30 WRs achieve each year) but be drafted at the top of the draft. That would put them among the top 10 to 15 WRs in the country, or approximately the top 10% of all #1s.
I don't think anyone would argue that Fuller's got a ton of work to do before he's in that class. Instead it's that that standard doesn't capture the full universe of #1s. Fuller is 19th in receiving yards, tied for 2nd in TDs, and tied for 27th in receptions. And he's done that as a sophomore. He's not looked like a big, dominant #1 doing that; he's looked like a very good, fast #1 with shaky hands, occasional concentration lapses, fantastic speed, and elite vision. He reminds me a bit of Kenny Stills. I'd call that a legit #1, just not an elite #1.