FWIW, we've also had professional usability experts involved with KDE many times over the years. E.g. the OpenUsability initiative, which KDE helped set up, was run by HCI professionals and conducted a fair number of user studies, produced research docs, and so on.

The difference perhaps is that OpenUsability didn't limit itself to working only on KDE (and also helped out, e.g. LibreOffice), that's why it somehow didn't get booked as a KDE thing and didn't become a similar anecdote people cite now.