If it's a hosted "thank you" dinner for friends who are doing a specific thing the host wishes to express thanks for, then, heck yes there *is* the pre-requisite that those who accept the "thank you" event invitation intend to do the thing for which the host is expressing thanks.
I agree that, in future, waiting to know who will be participating/doing something for which the OP wishes to express thanks will be one way to send a targeted evite, but I still see nothing wrong or off in the way this evite was sent, nor does a friend's misunderstanding of the intended scope of the invitation require the OP to change the intended invitation, which was conditional.
That, to me, *would* be analogous to a wedding invitation sent to a single person without specifying "and guest" and the single person misunderstanding and RSVPing for two. While misunderstood invitations can be awkward, a potential guest's misunderstanding does not preclude a corrective explanation from the host. Nor would such a correction seem to me rude or exclusionary, it would be a presumably polite correction to another's erroneous interpretation in lieu of an unexpected or intended change to the host's plans.