It’s not an assumption. It’s a pretty common observation. Employers don’t want “branded” designers, they want people who will work for their mission. Designers that focus on their personal brand either 1) tend to be pretty bad fits for employers because their goals won’t align, or 2) they tend to be misguided, believing a personal brand helps them out. This myth was to save designers in the latter camp. Those in the first typically aren’t looking for in-house UX positions.