The issue is for T-Mobile direct customers, which obviously their internal systems have access to. I see no reason why T-Mobile would have access to users accounts at another company…
"Google says that hackers may have accessed limited customer information via the compromised system, which includes phone numbers, SIM card serial numbers, account status, and mobile service plan data. The system did not contain personal customer information such as names, email addresses, payment card data, government IDs, passwords, or pin numbers."
It's something, but not perfect.
MVNOs do not roam on the carrier, however. The MVNO has a close direct relationship for wholesale access to the network. Roaming is a wholly separate method of access.
Of course you'll still be affected by SIM-swapping etc that just change how your number itself is routed.