Because you can send sats to an email address which has Proton Wallet enabled.
No address reuse, gpg signed btc address by your gpg private key but probably exposing the bitcoin addresses or in the worst case even the xpub to nostr:nprofile1qqsvc5xwftq6can0768j6eql250cv6v6k7cq7aakqe3l9meg75pt2ncpzfmhxue69uhhqatjwpkx2urpvuhx2ucpz3mhxue69uhhyetvv9ujuerpd46hxtnfduq3vamnwvaz7tmjv4kxz7fwdehhxarj9e3xzmny3tuzkj.