The app is owned by a consortium of Swedish banks, so you are using a bank app to approve the credit card / bank transaction.

At least in my experience, I only needed to authenticate with the app the first time I bought a train ticket, but I don't buy bus/train tickets in Sweden very often.

(But this authentication is annoying for trivial online purchases like this, so given the choice I prefer to use a physical card.)

It's not a government app, and you don't need it.

BankID (which is the one I assume he's talking about, and not Freja eID) is owned by several banks, but you aren't forced to use it. You can use a "bankdosa" (a separate physical device for auth)