Of course the forum can't control that and neither can your bank, but out of the two, only the bank should care and implement two-factor auth. It doesn't matter if your knitting forum account is hacked into, so two-factor auth is overkill.

Now, sure the password on that knitting forum might be the same as your bank online account. But the point is that only websites where your account is sensitive needs to add two-factor authentication.

I should have phrased my comment above another way: the solution to password re-use is not to add two-factor auth to a knitting forum, but to add it to the bank website, email provider, etc. anywhere your account's safety matters.

(I was thinking more from the point of view of the user: if they start to get worried about their accounts getting hacked, two-factor auth on the forum is not the solution, a password manager is)