I would have to imagine the system they have in place can not handle 302,330 requests/s even if you could issue them that quickly, but obviously that's not a security measure, just a practical limitation. There also may be other rate limits imposed which they do not advertise, but will kick in after 50 requests from any given IP address in a 5 minute period. I would assume if you could get away with trying 1,000 or 10,000 requests in a 2 hour period, that would be something you could demonstrate to them and have them listen. With what you've described now doesn't seem actionable, since performing 20 guesses is not the same as thousands to prove the exploit.
Also, I am not sure what regulations they need to follow as part of ICANN, but would imagine there is some documentation around what a registrar needs to adhere to for the security of transfer codes. This may be the minimum standards, and in that case, it would be on ICANN to update their guidance.