This might be true. An alternative explanation might be that such an employee changing the color of a button has probably millions of times more impact than an engineer working at a B2B start-up with three clients.
If I would be executive at such companies and I would have the evil intent of hoarding talented people, I'd, at least, give them a playground for experimentation. Maybe a new innovative product comes out of this. This hoarding has become a popular belief in the tech industry but I don't quite buy it.