1. You picked a profession which has a _shockingly low number_ of positions, and extreme requirements (including citizenship). This is basically a straw-man argument.
2. Astronauts have an incredibly high bar, and could come to the US if they wanted to anyways. Typically, astronauts have at least one (or multiple) PhDs. If you can get to the US as an astronaut, you are self-sponsor for a EB-1 or an O-1.
3. No, they wouldn't. Astronauts in the US must be (unless things have changed) US citizens.
Military astronaut candidates are U.S. citizens and commissioned officers with at least five years of active duty service.
And regardless, the point the parent was making is that even if it did, this would be ok, because the broader influx of immigrants would create more demand for all of these professions to the point where it'd be a wash.
If we allowed in 100,000 immigrant astronauts, I think it's safe to say they'd impact the hiring prospects of American astronauts. And they wouldn't consume enough goods and services to spawn new space ventures that create 100,000 new astronaut jobs.