I guess that is true, though they are biased, whether that is a positive or not.
There is however the bigger problem if there not being much of a point in making Streamers specifically test an Alpha when you could have an in-house Q&A. Consider for a moment the logistics of it.
Streamers do work by playing games for an audience. The NDA would preclude them from doing it as part of their work. So Blizzard is in essence asking a group of random people to playtest their game and do the work of a regular Q&A for free.
The only tangible benefit is that being public faces it is easier to single out and punish the streamers who leak stuff.