How would that affect the blood level if he was dead? Don't you actually have to be alive for the lungs to transport oxygen to the blood?
Red blood cells don't actually use oxygen because they barely qualify as living cells, so once the whole system shuts down it all kind of freezes in time for a bit. A RBC will only lose its bound oxygen if it exchanges it with tissue (does not happen after death) or if it's sufficiently damaged (at which point O2 cannot be measured). This is an extremely esoteric field and I'm far from an expert, but that's what I understand.
However, it also depends on what organ shuts down when. If the lungs shut down well before the heart, as would be expected in case of opiate overdose, O2 levels will drop extremely rapidly because the heart keeps circulating the blood. The blood continues to oxygenate the body as best it can, but that does not last long.
If an average person in good health (average spo2 99%+) holds their breath, they can expect to lose .1-.2% of their blood oxygen per second. George Floyd was a very, very unhealthy man, but even if he wasn't, if his respiration failed just 30 seconds before his heart stopped his oxygen saturation would theoretically rest at 94-97% postmortem, which is about the level you'd expect to see in someone who's just spent the last few minutes thrashing around. So oximetry would be totally useless.
You could theoretically -
very theoretically - attempt to figure out when he stopped breathing relative to when his heart stopped, but there are so many variables involved that it'd be closer to divination than science. I wouldn't trust any opinion based purely on postmortem oximetry.
EDIT: If his spo2 really measured 98% when the EMTs picked him up, that's really incredibly damning. There's absolutely no way, full stop, that someone who was just choked for 9 minutes would have that good of a reading. No way.