So first off, anytime they say "Here's a list of scientists who signed off on this, you should be incredibly skeptical. Who are these scientists? Are they biologists? Geneticists? Or are they random Meteorologists and "Political Scientists?" Are they even scientists at all? Or are they just random people the person making this claim is pretending are scientists in order to get prestige and attention? Hell, do they even exist at all? They could've just lied about that entirely. If they do exist, and they are actual scientists in a relevant scientific field, what does "signing off" actually mean? What did they sign? Did they actually sign anything at all? These sorts of con artists love to include a scientist's name on their bullshit for clout, without ever actually informing the scientist in question. Kind of hard to imagine that the scientist is actually signing off on something they don't even know about.

It's not some "top down" thing, most likely this is a con artist who figured out they can make a bunch of money selling "exclusive rights" to "study" their bullshit, and they've smoothed talked the right people into giving them a congressional hearing. That doesn't mean the Mexican government actually believes this 100%, just that someone there organised this in an attempt to legitimise these claims.

Politicians aren't experts on aliens. They wouldn't actually know anything more than a layperson about alien biology. They can be fooled by a con artist same as anyone else. Or they might not be fooled, they might see dollar signs and decide to get in on the grift, trading political clout and attention for money.