So the fuel was mixed in at that point?
That is what carburetors do.
They meter (add the appropriate amount) and mix the fuel and air.
I thought the airflow increased to the square of velocity?
Drag goes up with the square of the velocity, airflow goes up in proportion to the velocity.
The XP-39 had an intercooler that was too small. In part because they were trying to keep the weight down (XP-39 was at least 10% over weight) . However this causes a large amount of pressure drop across the intercooler. Read drag for pressure drop. Any radiator, oil cooler or intercooler (or air cooled engine baffles) is going to have air moving slower/at lower pressure on the outward side than on the intake wide. However a low pressure drop means there is only a small reduction in the velocity of the air moving through the cooling device. The larger the pressure drop the larger the change in the speed of the airflow and the more drag. However this is only one measurement. Is a 2in pressure drop over 4 sq ft radiator better or worse than 4in pressure drop over 2 sq ft radiator? wer get into the squares of velocity and the fact that the edge areas of the cooling device don't work as well as the core (inner areas) so it is not that simple.
On the XP-39 they were hoping that the intercooler would remove 50% of the heat added by the turbo. What they got was a 25% reduction in heat added in level flight and a 12% reduction in climb.
As an example lets say that the turbo added 100 degrees F to the intake air at 12,000ft. they were hoping to get that down to a 50 degree rise. What they got was a 75 degree rise in level flight and an 88 degree rise in climb. Since the hotter air meant they were running closer to detonation they had to limit the boost/power of the engine which hurt the performance.
at around 23,000ft you might have had a 200 degree F temperature rise. which is only partially offset by the 39-40 degree drop in air temperature.
Please remember that TWO stage supercharged Turbo aircraft and intercoolers were in their infancy at this time. Still in the cradle. While the US had built dozens of turbo charged planes at this point the vast majority had been single stage engines. Turbos added to an engine with no engine driven supercharger so the temperature rise wasn't as big a problem. There wasn't going to be a second supercharger adding another several hundred degrees to the intake charge.