As a general rule,
the puzzler decides, so I'm just giving an humble opinion:
If the answer is in the right direction but wrong, it should be open to all (a modified Cadillac?)
If the guesser obviously got the car but it is incomplete or slightly wrong, the puzzle should be locked for him.
My opinion is that writing "1978 Cadillac TAG Function Car" Maxiu proved that he found the right car, plane and simple.
The question was "What's this? 1 point for its correct identification" without a request for specific details.
On the other hand the Sbarro involvment was not a "detail".
. I should perhaps have locked it for him, but I thought he would be straight back with it
Yes, but let someone else get straight back with it sound not much better...
I felt the same with a Burney\Crosley Streamliner puzzle: I found the car but post an incomplete\wrong answer and someonelse fixed it.
...you can live with that!