I have to take exception with this statement. The turning driver can't just take a "still photo" of an oncoming car and compute how long they should have before the oncoming car gets to the intersection. Instead, the turning driver must observe the distance and relative speed of the oncoming car (a "video clip" in their brain) to determine the time until potential impact. It doesn't matter what speed the oncoming car is going...this rule applies at all speeds below, at, and/or above the posted speed limit.
I've heard many times about a stopped car pulling out in front of a vehicle in motion, getting hit in the back, and then saying "you were speeding". The obvious question then being "so you knowingly pulled out in front of a speeding car?" The car that pulled out is still the cause of the accident.
Having said all of that, in both cases, the "speeding" car's driver was overdriving their skills and/or the car's ability. If you're going to drive over the speed limit, you better have a reaction time in the top 10% of all drivers, excellent stopping power (like an MkIV Supra), and be paying 100% attention to what you are doing and all of your surroundings. If you can't commit to that, then the speed limit is for you.