Testflight will always notify you that you are participating in a "closed beta" as we use Testflights closed beta environment for testing.It has been communicated by Muldar as open a few times, but I think the confusion arose with yesterday's testflight emails. These were obviously from previous closed betas and haven't been updated for this beta. So yesterday everyone is suddenly seeing closed beta in their email and in the forum on topics about said email.
Bit of an annoyance and it has caused minor issues in the past.