2 platforms similar to FiveM, use the gta5 executable itself to track playtime on Steam, and FiveM uses this “source SDK” 
since the creation of the fiveM platform 2014 …
Why don’t they use the game’s executable to track the hours?
It’s not impossible to do this; in fact, it is possible.
There’s an experiment to use a new https://steamdb.info/app/2676230/ appid so it will likely be using that one in the future, not the main GTAV one.
I saw this appear on steamdb the other day, is this something we can activate ourselves?
And is one available for RedM as well? Perhaps it would solve the issue I have with redm counting for my rdr2 playtime, see RedM game time counts towards RDR2 on Steam
The steam integration rework is on beta and canary update channels for both FiveM and RedM. Although it is worth noting that currently upon joining a server it will revert to the old source sdk base appId for authentication purposes. It may or may not resolve the issue you mention, I would recommend switching to beta/canary and checking if that is still the case.
