I'm not a lawyer but I can't see how the insurance contract can be changed retrospectively.
The point is, anyway, a mortgage holiday simply defers payment (at a cost of more interest) whereas a rental default is/maybe an actual loss (and not just a deferral).
If they are saying that an agreed deferral or rent reduction prevents the loss being treated as within the rules for a claim, then I guess that's debatable. Might be within the letter of the contract.
Be interested to hear informed views.