It's not an actual issue. If you want them to check it out, that is your right. They will probably just drain and refill. If they refill it correctly, you will be fine. You will also probably be fine if you don't do any of that.
I'm the wrong person to ask about dealers and warranties. ;)
My cars haven't seen a dealer for like 20 years. 95% of the time they just make things worse.
If you are going to add a flex fuel sensor you are 95% of the way there to having full flex fuel. It's just Power, ground, and CAN hi/low.
TLDR, if you are going to add a sensor, go all the way and do full flex fuel. Just not Visconti because he is a jerk and his customer service is horrible...
OEM takes air from the outside. The Supra is turbocharged. The actual charge temps pre intercooler are more like 300F+. The IAT you are seeing on that gauge is most likely after the intake manifold intercooler.
I'm assuming it was 70F+ out that day.