I don’t know what exactly they have said, so I can only comment on what I see before me here …
Not primarily for finding bugs, no. Beta testers are not supposed to go looking for corner cases. It’s not the primary concern.
(But if you throw enough beta testers at a new feature, corner cases are bound to show up. Just all the combinations of hardware, system versions, hero rosters, play style … whatever the primary intention, beta testers cover a lot of important ground just being themselves. Not looking for bugs. Just stumbling upon them.)
QA though? I’ve worked with QA. Beta testing is part of QA. And “feedback from players on features before release” is usually primarily sought for QA.
(And unless it’s phrased so as to suggest “feedback on the concept designs of new features”, it should as it stands also cover “feedback on the implementation of new features” – i.e. bug reports.)
So if beta testing is not intended to be primarily for QA, I’m missing a few pieces of this puzzle.