You do have a point. The only times I have noticed getting the "good move, but not the best" message is in mating sequences when you play a move that requires one extra move to mate than the minimum. It's worth noting that the puzzles are automatically generated, which may limit how well one can automate when the "good move, but not the best" is appropriate.
The lichess puzzles have been good for me for calculation and making sure to notice all details of the position since the lichess puzzles usually require the absolute best move to get the puzzle correct. While sometimes winning moves are considered wrong, I don't mind because there are many times where finding the most accurate move is necessary, and the lichess puzzles test whether you can find that best move even though it may not be "necessary" in the given position. Obviously lichess isn't the best site for puzzles, but it does have value, and the automatic generation will give us an endless source of puzzles. I also do puzzles at
chesstempo.com.
Anyway, I'm not really meaning to argue against your point. Just pointing out a few reasons why I don't mind the way grades puzzles.