Using an internal HoloLens prototype is the same as using their equipment or infrastructure. It belongs to them and you're using it.
That's exactly what I'm trying to say. If I use the publicly available devkit I expect that they have no claim.
If I use an internal prototype they have claim.
Sorry if I wasn't clear enough before.
TL;DR If you could not have built the product without being an employee of the company, it probably belongs to them.
eg. Maybe you use Microsoft's latest internal only HoloLens prototype to build a product. You couldn't have done that if you weren't a MS employee - so they can claim it. But if you used the dev kit and publicly available tech and features then they shouldn't be able to claim it.