Again, we can't apply real world logic as something valid to a fictional work. I know every person is entitled to their opinions and I respect that, but take this pov into account. Even Jin (Yuji's father) talk about the fact that babies might remember more than what we think they would. Some adults yelling that his mom died isn't something light. Even if, again, you don't ignore the fact that Itadori Yuji isn't a normal person at all. His mother was possessed by some mysterious entity at that time and he is the perfect suited vessel for a thousand year old shaman turned cursed spirit.
And as I said, some researchers do say that if it's a traumatic enough moment, a baby might recollect some memories.
One more point is that she/he might asking in a sarcastic way. Or questioning like, y'know, when someone say something that isn't truth or right and you turn to them and "what are you talking about?". I'm sure that's the kind of tone she used to her question, like she wasn't dead and in fact was there in front of him talking. Grandpa was yelling or else the father wouldn't ask for him to low his voice. I don't see how that was speculative.
As you said, Gege isn't the first person to wonky science in fiction. A lot of authors and directors mess with it in manga, comics, movies, animes...