I was wondering the same??? Maybe S650 is the designation for the sheet metal refresh in 23MY and S750 is the new platform? Or he just misspoke...So, are we skipping S650 and going straight to S750 as the codename for the 7th Gen Mustang?
Taken from a Q&A with a Mach E Electric Vehicle Engineer:
"Q: Does this mean the Mustang is going to be a crossover from now on?
Nope, the new generation Mustang coupe is in development, actually! Going to be a special one, too.....
......because of the Mach-E we get to keep the S750 program."