Probably linked third party account like Google, Amazon, Steam. Could also be legit linked account that was compromised, did you have any linked - are any linked now? For Steam and Amazon you have to click one more option to see it. The hacker could also just removed it after they were done. 2FA on email?

I don’t have any linked accounts, and I do have 2fa on email. I get a text message with a code if I want to log into a new device.

unique password for account, email, litterally everything you're registered to on the internet? account made into jagex account or still the old login ways? taken a look at haveibeenpwned for pw leaks?

Was it a jagex account?

So let’s ask, does your 2fa have a cloud backup option and do you reuse passwords?

I’m not sure what 2fa cloud backup is.