You're both correct in you assessments. To clarify, the use of the shifter as it's intended to be used with the clutch is not a bug. The use of the shifter in the way it has been described here is a bug and to use it that way is exploiting the bug and is against the SNAIL OLR.
With that being said and as
@soundtiger95 said, there is no reliable way to determine, without a doubt, if someone is using the shifter as intended or if they are using it in a way that exploits a bug.
This is the spot we are in right now and there isn't going to be anything that can be done about it short of an update to the game that fixes the issue or someone coming up with a way to check how the shifter is being used. I will say to those that are exploiting the bug that your fellow drivers know you are doing it and it probably doesn't reflect well upon your character and integrity.