It’s a bit of a moot argument though.
In order to ride fast on my HT, I end up taking completely different line choices than I would on my DH bike.
If I was to try and transfer that “technique” of line choice to my riding on the DH bike, I wouldn’t necessarily be picking the most effecient lines for that bike.
So, riding a hardtail well will make you a good rider on a HT, but not necessarily on a full suspension bike.
The reverse is also true when trying to apply a full suss riding style to a HT (although often with more sketchy results!).
I think what people mean whenever this question comes up is “does being able to ride the same section as smoothly/quickly on a HT as someone on a FS bike make you a better rider?”
Even to that question, I don’t think it makes you a better rider, it just means you pick efficient line choices for a HT. You might try the same section on the full suss and totally mince it, therefore you’re back in the crap rider category 🙂