That's a small low profile tire...it would be a 205/18-15! A more typical (but still low profile) 205/50-15 would have about 1/3rd of a pound--still inconsequential, but more reasonable
It's unsprung weight, and it's in the wheel, but at the same time it's not
exactly rotational weight either because the air isn't going to rotationally accelerate right along with the wheel and tire, it'll lag behind. Like when you rotate a drinking glass and the ice and water inside don't really move.
But, for sake of argument, lets assume the air mass is actually locked to the wheel and rubbber:
A fun math exercise would be to take the 1/3 lb, treat it as a evenly-distributed torroidal mass that its inside the center of the airspace of the tire, and measure how much power would be required to accelerate it up to whatever wheel RPM you have at the end of your best 1/4 mile run, in a time window equivalent to your ET for that run. Go ahead and add in the linear acceleration along the track for a true worst-case scenario.
If you figure that out, I would love to know what X is in the statement "During my best 1/4 mile run in the Skyline, X horsepower was used just to accelerate the air in the tires."