I think they’re referring to actual implementation, which in their experience would require some sort of massive architectural stupidity to produce a slow UI on desktop.
And you’re referring to your (and most other people’s) daily experience, which is of a major software firm producing daily used software with a super slow UI on desktop.
With a little cynicism, these two views are quite compatible.
Some things do run very quickly, for sure, but so many of the high touch pieces of code out there from big name corps have some of the worst performance. Hundreds of millions of people use Teams, and many of them use it a lot throughout the day. You must just be getting lucky in what apps you use on a regular basis.