No, there are lots of mints. You can send your nuts from one mint to any mint via lightning transaction, even to a mint you own. Cashu wallets let you have many mints with separate balances. You can use nostr:npub1kvaln6tm0re4d99q9e4ma788wpvnw0jzkz595cljtfgwhldd75xsj9tkzv + Amethyst and that uses the minibits mint. npub.cash is one solution primarly for ios users and desktop users as nostr:npub1dx5q2el8nd4eh3eg9t2e25fd7zuqg7zxz6ldkc3uzgh66ss2yc6st288sj doesn't have the same ln to ecash bridge that minibits has.