The general rule is if someone receives a save-the-date card a formal wedding invitation should follow. Regardless of what they told you you should still send them an invite. What if their reason for not attending changes?
Knowing they can’t make it can also allow you to move some B-list guests to your A-list. You can either send some B-list guests an invite when you mail your wedding invitations or just hold them until the no RSVPs arrive.