Unless they can place him in the car they cannot give him a sobriety test.
They can place him in the car either by witnessing him in the car i.e. they pulled him over while driving, they found him sitting in the drivers seat. Alternatively they can try getting him to admit he was in the car. The cop tried to sneak up on that but couldn't get him to say he was in the car. The car wasn't registered in his name so they can't give him a breathalyzer any more than any other college kid standing hear by.
Remember the fact that he was driving came out later,not that night.
The seat being too far forward for Simpson to be driving is not exactly evidence, either. It is very plausible that the seat was adjusted from the time the crash occurred to the time that the police showed up. That is pretty terrible investigative work.
That wasn't investigative work. That was step one in trying to get him to admit he was in the car. Giving him some wiggle room to say he was there but not behind the wheel. If you can get him to say that then you can press him about who else was in the car and who was driving and where did they go. Then maybe he either admits he was driving or he throws a pal under the bus by lying even more.
And to be clear I'm not making this stuff up I talked to my daughter, a police Sargent and a son-in-law a police detective about how this works in general and with this case. And to be further clear neither of them are AAPD they are Washtenaw County.