Not true.
They have the authority to rewrite the rules up to the day before the convention.
This is the test to win and then they will move the goalposts.
Problem is, they risk losing the House and the Senate if they do it, and they risk losing the House and Senate totally two years later if they do it along with Governors, state legislature, judges, etc.
The problem is the GOPe can change that rule prior to the convention.
The rules committee meets again before the convention and will change the rules to whatever serves their establishment purposes. Count on it.
Rule 40 can be waived