Maybe it works because there is no english word written 'ba', and in fact no english word that's 'b' followed by vowel(s) and nothing else, except for 'bay'. So there's no other word it could be.
But yeah, the extreme form of gregg shorthand definitely seems, from the OP, to rely on lots of contextual knowledge for it's concision. That seems to be it's strategy, eliminate letters where contextual knowledge will suffice to reconstruct them.
> and in fact no english word that's 'b' followed by vowel(s) and nothing else
"beau", "bi", "be", "bee", "bao", "boa"
came to mind off the top of my head and are all in the dictionary; there are probably others. Of these, "be", "bee", and "boa" should be pretty non-controversial both in terms of "word" and "English".
For the original issue, "ba" pronounced with the actual letter names does in fact sound like "b-ay". Whether that's what Gregg shorthand does, I can't tell.
But yeah, the extreme form of gregg shorthand definitely seems, from the OP, to rely on lots of contextual knowledge for it's concision. That seems to be it's strategy, eliminate letters where contextual knowledge will suffice to reconstruct them.