the rebreather masks are something small, same for air generators in the suit, it's not something big and silly like the Mon Keigh. though there are a couple of differences, kabalite armor is like the Mesh armors of the craftworlds. We know for a fact those are closed up.
The suit is still hermetic I think, the armor goes on top, but there's the leather suit underneath. I'm sure not not regular leather, some kind of weird eldar suit probably, might be solid enough to be hermetic. Kabalite armor might be sealed hermetictly, but Wych suits & covenite armor certainly isn't.
We know that Corsairs are specilist space fighters, they all wear masks, if you check their model range (FW), it's all with helmets and the suit and everything is space worthy.
In fluff it's mostly Kabalite that fight space battles and space raids, I'm guessing Kabalite armor is suited for space battles, though it's not fully confirmed.
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