How? He didn't exist yet so there was nothing to link the spell to him and he didn't even have a true name yet. The person who cast the curse didn't even seem to know he existed
For that matter, an entropy spell powered by an Outsider and a human sacrifice is kiiiiiiinda overkill to kill an infant.
If someone really, really wanted less-than-five-minutes-old Harry dead, there are so many easier ways to do it.
As for what Lea said? She said "near" the end, not "at" the end. Probably, she's referring to Maggie's change in viewpoint in the last years of her life, not literally her last words.