I think the more worrying attack vector is when you now use the account number to try and Allowlist a principal from that account in some policy in your account

I think the more worrying attack vector is when you now use the account number to try and Allowlist a principal from that account in some policy in your account. If the principal doesn't exist in the other account you'll get a role/user not found error! Presumably you could use this to find real principals in the other account.