I guess I just don't see why "hubris," though not to parse words. I don't think Sinner seems like an arrogant guy. I don't think he didn't feel it was serious, and I'm not sure why you think so.
It's easy to judge from the sidelines what someone "should" have done, but I personally can imagine at least a period of discussing when and if best to disclose ahead of the judgment. Ill-advised to sit on it? In retrospect, yes, probably. And why did he fire those guys only when it became public? That seems a bad look. Maybe he is dirty, but, if so, why not build a story and get in front of it? If it was all inadvertent, maybe he and his team were hoping that the ITIA would see there was no ill-intention, and that it might never come out. The problem is that there is taint, once it does, no matter the final judgment. If he was clean, he may have hoped to get past it without the publicity. But I wouldn't call that hubris, I'd call it naïveté. And I don't think Cahill "canonized" him. He said he was a good kid. A decent person. What would you expect him to say?
You get tarred with this brush, and some will never forget.
@Front242 is an example. Hell, when Sebastian Korda started coming up, Front had to bring up his father's doping, and wonder about him. One could see why, if you thought the whole thing was a mistake, you might hope it could pass unnoticed.
That said, and looking at some comments above, what happens if WADA wins their appeal and Sinner gets a 1-2 year suspension? Meaning, what happens to his US Open title this year? Just...oops, oh, well? Tennis needs to think about that. And I think Italian tennis has a bit of soul-searching to do, too. Attention has been cast in their direction, and they need to think about how to deal with their PR problem, and to eliminate any whiff of taint.