I don't think Dinsdale is disputing that a mororcycle would need to do 125mph to catch up with one doing 100mph. Isn't his question more to do with that fact that the Police are doing 125mph in the first place. I think he's trying to say that instead of one dangerous motorcyclist (doing 100mph), there would be two potentially dangerous motorcyclists. Therefore, indicating that he thinks it would be better if the Police didn't pursue and the first motorcyclist was left to cause havoc on whatever road he chose to ride on !!
By the way, Police motorcyclists ARE highly trained and are usually involved in bike clubs etc in their own time. It's the PC's who are passionate about bikes who end up working with them in their traffic depts.