This argument applies just as well to libertarians who oppose "regulation." There are some truly insane libertarians who want all regulation gone, but a lot of people who say they are opposed to "regulation" really mean that they want to add more barriers to adding regulation, and repeal some known-to-be-problematic regulations. I'm sure that when this person says "ethics" is holding back scientific progress, he means the latter. To assert he just means getting rid of "ethics" entirely is absurd. There is only so much detail you can put in a tweet.

So if we put these extra pair of legs on babies then they can stand in more extreme angles making them better at construction at a time when there is a housing shortage

And we already have a safety valve for when conventional ethics is standing in the way of vital research: the researchers test on themselves.

https://en.wikipedia.org/wiki/Self-experimentation_in_medicine

If it's that vital, surely you would do it to yourself?

It's not terribly common because most useful research is perfectly ethical, but we have a good number of cases of researchers deciding that there's no way for someone to ethically volunteer for what they need to do, so they do it to themselves. Sometimes they die. Sometimes they make very valuable discoveries. Sometimes both.

So the next time someone wantz to strap someone to a rocket engine and fire it into a wall, all they have to do is go first and be part of the testing pool.

Best I can do is generalization