Yeah he said the IAT was a nice to have.
Think about this very carefully.
SD means "speed density"
Speed density is named as it is because the tuning tables for AFR and timing have an X and a Y axis.
One of them is speed (RPM) the other is density, as in air density inside the intake manifold.
Density needs 2 sensors, one being temp, the other being pressure.
You can't determine density without the IAT sensor which should be mounted IN the manifold, not before it.
Now your wannabe tuner / parts fitter might tell you that you don't need it and you just lock in a fixed number.It's several steps backwards from having a factory ECU....The factory ECU measures flow, temp and pressure all in the same place and that gives the air mass. SD determines mass from density, RPM and VE, so when you're tuning SD you're putting in numbers that correspond to the engines VE at various RPM and manifold density points.
A turbo pumping out say 2 bar of boost will have a typical outlet temp of about 130 deg C, that goes through an intercooler of continuously varying efficiency.The 130 C at 2 bar might temporarily drop to 60 C through the intercooler but it'll creep up with time. What your manifold sees in terms of temp can be anywhere between maybe zero c on a cold morning all the way to 130 with 2 bar boost.
That means that at a constant boost level the actual density will vary a lot.....so your tune will change with the weather as does the efficiency of the intercoolers.
Not only that but most fitters fit the AIT before the throttle body, that's wrong too because a partially closed throttle drops the pressure but it drops the temp too, That's why the throttle is heated, to stop it from freezing with ice on a cold morning.It might be OK for a drag car that only sees full throttle, useless for anything else.
All factory SD installs have both sensors in the manifold and in fact a lot of modern cars have both sensors built into the same package for ease.
Prior planning prevents piss poor performance.