From there statement regarding the segwit2x fork:
"Xapos policy in regards to forks is that, when there is a fork, we always follow the chain with the most accumulated difficulty and we make the minority chain available to our customers for them to sell or withdraw from Xapo. "
So, if someone changes the hardcoded rule about the limit of 21,000,000 btc max. it would be totally fine for XAPO as long as the new chain has the most hashrate.
This is why segwit2x is a serious threat to all cryptocurrencies. We could no longer argue why some rules are irrevocable.
There is no mention at all of exceeding the cap of 21m btc, you're simply plucking that out of thin air based on something unrelated to the cap.