If the OP cannot even recall a second bird being present, that is the defining issue for me. Don't tick it. If he recalls a second bird being present but didnt really look at it, then I suppose it is ok to id it belatedly. Its your own choice of course.
Personally I don't think it is critical to be able to i/d a bird at the time, as long as you know you have seen the bird in question. I saw the Filey Brigg Two-barred Greenish Warbler, but left not sure what it was, possibly thinking Arctic Warbler. I was the 2nd birder to see it, and neither of us initially were sure (though in fairness to the finder I think he possibly harboured that opinion). But it is on my list (as a subspecies of Greenish obviously). I saw it well.
I hear of some birders not counting the Yorkshire Amur Falcon because they thought it was a RF Falcon at the time. For me, that is cheating yourself, unless your list is what you identified, not what you saw.