True. Then again, Harry knew when he taught Draco that one of the uses of a Patronus is to carry unfakeable messages, for which you need to know exactly what the other person's Patronus looks like. Also, it's the snake on Lucius's cane, which we know he paid attention to. If he recognised Draco's snake as that snake, it would set it firmly enough in his mind that he might recognise it when he saw it again.

File under "more evidence needed".