Well spotted. RF2 rear tyres are too cold compared to front tyres. I go to early 90's prototypes example again (my favourite era). Real tyre temperatures after some laps on a high speed track (Hockenheim). Goodyear R430 and R240. Fahrenheit degrees and psi hot pressures. Sorry, but this is impossible to achieve with standard RF2 tire parameters. Only after lots of trial&error method, intuition and luck you can find some numbers which provide desired result. I hope developers don't take this is as an attack to the software, just as feedback to improve the issues. When all issues we are talking about are solved we will love Rfactor 2 at least as much as we loved Rfactor. (today it is still a love/hate feeling )
I'm a big F1 fan that happened to watch the quoted interview. Maybe it's just a question of Frank Dernie trying to keep things simple/useful.
Mechanical grip, may not exist except as a term. But it is used to describe traction provided by components OTHER than Aero. Suspension, tires, springrates etc all comprise what is called Mechanical Grip. Non-Aero might be a more appropriate way to describe it.