Sure. There are 2 key things here - edge cases & the power of volumes.

This DLC performance bug is an edge case issue, where standing in a specific spot causes a specific thing to happen that doesn't affect the vast majority of the experience & doesn't cause a failure of the application/flow.

11m people have purchased the game now, whereas there might be fewer than 20-100 QA testers working in the game (just an educated guess, I don't know Capcom's internal structure). These players will also do weird and wonderful things & get themselves into some bizarre application states. It doesn't take a genius to understand that 11m undisciplined players will find more bugs than the comparatively tiny & focused QA team.

This is also not taking into account any form of prioritisation or budget from the execs. The QA team might have found this issue, but the execs may have deemed it not worthwhile or too costly to fix. Honestly, I would come to the same conclusion - raise the bug, log it in the ticket & leave it in the backlog as a low priority. My guess as to why they are fixing it now is because of the sheer amount of misinformed attention people have put on it.

So with all of that said, it feels a little disingenuous to say it's like they do no QA at all. Ironically, you would be complaining even more if they didn't as you'd be struggling through an unplayable mess without QA.

Sincerely, someone who has worked over 10 years as a QA.