If I'm reading that right, that's 12 expected goals and assists in 38 games?
For context, same website, Diaby (who, let's be honest, hasn't been great) has 9.9 in 28 games. So he has 10 more games, to get 2 more expected G&A to have a season better than Buendia. And he's been pap this season. https://fbref.com/en/squads/8602292d/Aston-Villa-Stats
To clarify once again, I like Buendia, and I do think he'd offer us something we don't have at the minute, but the train of thought that our season would have been much better with him in seems a bit odd to me.
(Also, fully appreciate last season wasn't as good as this season, didn't win as many, didn't score as many, but that's still not enough to convince me)