@Monkey:
Neko's comment was unrelated to the little side commentary Momo made on an already ongoing conversation.
And uh do you not interact with children ever? Are they generally strongly aware of every adult they've met before and very conscious of the passage of time? No not really.
Maybe, or maybe he also added to the exchange Usopp and Momo had.
Momo mentioned it extremely casually, like he was badly recalling memories and confusing things. Not to mention saying something without really thinking it through.
This is somehow a more outlandish idea then time traveling Gold Roger to you?
Why does it have to be either time travel or Ace? There are numerous posibilities. He saw a snail recording of Roger, he saw a play about Roger's adventures as a child, his growth was suspended or he was in some sort of cryostate for a couple of years for whatever reason. The Ope Ope was used on him causing him to stop growing or to growing a lot slower…
Hell, it might even have something to do with his devil fruit. Perhaps Vegapunk derived it from the samples of the dragon that was born on Roger's ship, in that egg we saw and Momo somehow confused it's memories with his own. Somehow? Maybe the ability to hear the voice has something to do with it. Maybe it's connected to the PH dragon whose name was supposed to be important.
Some kind of variation of the original Momotaro story, but with egg instead of peach.
Then there's a fact Vegapunk's devil fruit is supposedly a failure, but seems to be working fine. Maybe Momo's transformation is more complicated than that.
Seeing what Greg said, though, I doubt it's either of these things.
You realize Ace and Roger being blood related have some y'know, visual similarities? That an uninformed half interested kid wouldn't be an expert on? You're talking like Momo is a One Piece fan following the series and researching everything on wikipedia at the same time lol. As to being sad, it's pretty obvious Momo even in his confusion didn't feel some heavy connection to the person he met (why would he if it was a quick visit from Ace or whoever?).
Visual similarities between 17-20 hairless faced man and 40-50 year old mustached man a small child would pick up on? Stretching it thin there.
Roger seems to be a pretty big part of Momo's family history, so I imagine he'd know some stuff. Enough to not confuse things that bad.
I'm more inclined to believe it would have been someone else connected to Roger.
I can see it being Ace maybe if he overheard WB or Oden, for example, talking about Ace being Roger's son, meeting him afterwards and remembering wrong, but it still sounds contrived. As if the line was dropped only to create a plot line that doesn't go anywhere.