I suppose a hole-in-one is a subjective thing since I don't believe it's covered in the Rules of Golf. I just read a story about a guy who teed off on a par 3 and knocked it into the water. He then re-teed and knocked it in the hole! He is calling this a hole in one. Hmmm???
Personally (subjectively) I feel an authentic hole-in-one, in the spirit of its intention, is that you score a one on a hole, in the course of play on a regular course with par 4's and par 5's being part of the course. On an executive course there are no regulations as to hole length. Taking this to the extreme, what if they had a 20 yard hole? If you sunk it off the tee is this a hole in one?