In my book, the #1 or X is the guy that lines up outside, on the line, often faces the defenses best CB. Because he is outside, on the line he often faces press coverage. The other positions are more flexible, but typically the #2 or Y lines up outside but off the line as the TE is on his side. The #3 or Z is the slot guy.
Wallace couldn't handle being the X and wouldn't play it. So when Diggs was playing, he had to play that position and started struggling later in the year with the press coverage. IIRC, he acknowledged that and worked on it and is much better against it now. Last year I doubted Thielen could handle the X. I thought he was too small, not physical enough. But he played it and even thrived allowing Diggs to thrive as well.
Now our offense has changed, I think we run more 3 wides and actually Treadwell plays the X spot with Diggs at Y and Thielen at slot.
But as far as being the go to guy, Thielen is getting it done without Diggs drawing attention. But he is playing slot, and likely facing the nickle CB rather than the top guy.