I'm not sure how this doesn't qualify as a beta. During the alpha stage of development a company is voicing and designing the product. The beta period is often used to test implementation and build quality. In software development it's when you find and squash any bugs that are exposed, in hardware testing it's much the same idea. By sending out a handful of beta units to testers they can find out if there's any issues with the construction or durability of the design. And as was mentioned before these are expensive headphones and people may contemplate keeping them without prior authorization. A deposit is a great way for HiFiMan to make sure they receive the beta units back from testing.
I'm going to agree with Alchemist and say that anything shared in PM between potential testers and HiFiMan should not be shared in a public forum such as this. By replying to the initial PM and confirming interest in being a beta tester, even if you choose not to continue upon learning additional details, you've agreed to what is effectively a verbal non-disclosure agreement. When anyone is contacted about potentially testing a product there's an unspoken agreement that details should not be discussed unless explicit permission is granted.
Just something to think about moving forward.