Their password should already be blank but I guess it depends on the exact Steam implementation. We have an authentication handler called
XF:NoPassword
which is what should be given to users who have registered through a connected account and never set a password
As a proof of concept I just registered a new account via Twitter here and then from my admin account set it to security lock.
When I log back in with Twitter, it works as expected:
View attachment 253233 View attachment 253234
After receiving the email I just need to set my new password:
View attachment 253235
If your experience is different are you sure you tested on account that has never set a password? Even if you register through a connected account it is possible to add a password to your account down the line by going to the account/security URL.
But for an account that has never had a password set the experience should be as I set out above.