I think it’s legal. Here’s the definition of start:
A boat starts when, her hull having been entirely on the pre-start side of the starting line at or after her starting signal, and having complied with rule 30.1 if it applies, any part of her hull crosses the starting line from the pre-start side to the course side.
So, it doesn’t matter where they were when the gun went off- as long as they get the whole boat on the pre-start side and then cross the starting line they are starting legally.
The exception is in the unusual circumstance that the committee flies the I flag. In that case (rule 30.1) if you’re on the race side of the line or its extensions before the start you must go back across an extension to the pre-start side before starting. That means you’d need to get below the RC boat (or pin), then loop up like normal. Even with the I flag up if you’re still on the pre-start side of the extensions at the gun you can dip the line - it’s only if you’re on the course side of the extensions at the gun that 30.1 applies.
The committee can fly the Z flag, U Flag, or Black flag too, but those only apply if you’re in the triangle between the pin, boat, and 1st mark in the minute before the start.
