The Closed Alpha playtest isn't an invitation to publicly review, it's an invitation to playtest. They're trying to gather data and feedback on an inherently feature-incomplete and unpolished game to help with development. There are going to be private channels for feedback and the playtest data itself is like feedback so public channels are redundant. Obviously Marvel is also just trying to dodge criticism, but that's not a mutually exclusive reason.
Yeah this seems to be something people are missing. These tests sometimes prohibit all reviewing and commenting in their NDAs (including positive ones). It's a playtest, not a beta, review copy or pre-release.