For a car comparison, it's easy to argue that going faster is less safe, but that's not enough to explain why a speed limit for a road is specifically 35mph, and doesn't tell you if 35mph is correct for that road. Maybe it should be 25mph or 45mph.

The rest of their argument is making a bunch of assumptions with no evidence shown. They're assuming that the current line is exactly enough to determine the level of danger to the point that you can get informed consent. They're assuming that the current line is an exact match to what's ethical.

But maybe that's not true. Maybe the FDA is 50% too lenient. Or maybe they're 50% not lenient enough. This commenter is failing to make a case for the current regulations being just right. They're making a strawman about changing the regulations a huge amount, jumping all the way to nearly unchecked experimentation. Of course that's bad, but that's not what the neurosurgeon actually said.

It does nothing to explain what is ethical or not. Have someone who's worked in preclinical testing and clinical trials, my impression is also that they don't have firm understanding of what the current status even is. It isn't a hard line, but negotiated with the FDA on a Case by case basis.