In my opinion and limited understanding of xGOT, I think it is completely the opposite. From what I gather, xGOT is accumulated, meaning that many shots that are normally of a poor xG statistic, will accumulate to a high xGOT. If you for instance are bombarded every match with shots outside the box, they will accumulate to a high xGOT, but you would also expect the goalie to save arguably all of them. Thus, facing 20+ shots every game actually inflates his stats. 10 shots with an xG of 0,20, will actually inflate Onana's stats quite drastically. Using the eye test, this also rings more true. I cannot understand that anyone watching him play can rightfully argue that he is top three goalie in the league in terms of shot stopping. At the very best, he is average - I'd argue slightly below average. In particular, low shots seems to be an issue with him. His technique there has also been called out by Schmeichel for instance.
There was a piece on this a few years ago where Lloris went from having very good stats to suddenly becoming one of the worst in the league. Delving into the stats meant that he hadn't change, but the style of the team had changed so that he faced far less shots, but the ones he did face were often one on ones that he was not expected to save, but accumulated he had a negative xGOT. The next season with a different manager and style his stats went up again.
Guessing, I think manager of opposing teams tell their players to shoot because he will let some in - and we do. Remember when DDG came to the club and let in a good few very soft shots? Attackers started shooting left and right at him, and seeing as some went in, they kept on doing it. When he became DeGod, they stopped as it was pointless. With Onana, players may very well think that a half decent shot has a chance of going past him still, and so they keep on shooting even if xG is a little on the low side.