While INS is not simple, it's also not that difficult to implement. As even the cheapest drones that can hover show new MEMS sensors are accurate enough to work with a proper filtering.

I did some prototype INS system as my master's thesis 10 years ago, the code was quick and dirty and even then the accuracy was like 30 meters after an hour of walking around with the device.

This also depends on the plane. For larger commercial applications, you need to have INS. You want to use a combination of sources ideally, since INS is accurate for a shorter period and GNSS/GPS for longer periods(often GPS/GNSS is only updated every second). Plus a lot of newer units have a much smaller drift. This FOG has a drift of 0.1 deg per hour, which is quite good (https://www.advancednavigation.com/solutions/spatial-fog-dua...).

I think they may have been joking since INS - is inertial navigation.