Same with pets! I would have long "conversations" with my vocal cat...

That stops being nonsense a lot faster than people think it does. Mothers commonly report knowing if the baby wants food, feels pain, or wants to be held very early. Then at about 6 months they're trying to learn language, they'll repeat sounds as best as they can. And at a year they'll have their first word.

So, yeah a 6 month crawler should be getting full sentences but in baby tone.

Kids perceive a lot more than we might think. I know my parents made lots of well-intentioned, passing comments that were nothing to them but stay with me decades later.

While I agree with you about the funniness, I worry that a kid might justifiably feel condescended to by that response and thus lose trust in the responder, an authority figure - especially if that figure is a parent, which is to say, a person they have to trust as an implicit safe figure.

I want my toddler to feel free to say anything to me, be it gibberish or a deep and well articulated philosophical point, and know that they won't be mocked for it. That's how they know it's okay to explore and, if they wish to, share their thoughts. Even if their thoughts don't make sense to me.

Teasing a kid isn't inherently wrong, but even before they're articulate, your response to their words - or gibberish - matters.

Conversely, it's also very funny to respond to self-important adults babbling nonsense with baby talk.

Another good one is to suddenly look frightened and stammer out h-how could you know that".