The difference between a "true RMS" meter and a plain old garden variety AC meter is the ability to accurately measure the voltage of non-sinusoidal waveforms, for example a square wave. "Normal" meters tend to under report the values of non-sinusoidal signals, so you might see readings that were lower than expected. This could lead to misdiagnosing problems, I suppose.
To really answer the question it would help to know where GM thinks a true RMS meter would be used. I'm guessing on engine or wheel sensors, but I don't have any experience with GM vehicles – all of my SAABs were pre-GM.
I suppose it could also be the GM sees a need for techs to be able to make accurate non-sinusoidal readings in the future – perhaps for electric cars, or because of new sensors – and so they are trying to prime the pipeline by encouraging techs to buy the tools GM expects the techs will need in the future.
Fluke has a page on their website that explains true RMS.