I've not seen that case mentioned. After final payment all bets are off and Celebrity is going to sell the remaining inventory to whomever they can. If you need an accessible cabin you surely should be booking before final payment. This is the case of any limited cabin class, it's not guaranteed to be available as a last minute booking.
In the case you quoted it was assigned as a MoveUp upgrade obviously after final payment.
The other case you mentioned was "My TA said “look I have accessible cabins available on 1/2/25”". You TA has visibility to all available cabins. It was them not Celebrity who was claiming it was available to you. I'm very sure if you searched for the category on the Celebrity site without the "accessible" check box checked you wouldn't have been offered an accessible cabin.