That's what I thought too until yesterday. But under Primal settings there's an option to add "Other wallet". I simply scanned the Nostr Wallet Connect (NWC) QR from Minibits and was done. Works on desktop as well as mobile. Also works for Amethyst (mobile)
Discussion
Oops, replace "minibits" with "Cashu". Minibits is the mint I use in Cashu