It is true that premierships are not recognised for assistant coaches.Bomber Thompson was senior assistant to Denis Pagan for North's 1999 flag.
Does the footy world really go oh well Bomber's coaching success really started with his 1999 flag at North?
No they don't, for good reason.
I would have said that Hardwick has 5 premierships to his name (2 as a player and 3 as senior coach) and not really counted the one where he was the assistant coach for the Hawthorn 2008 premiership.
Even Wikipedia lists the premierships as a player and senior coach as achievements. Doesn't seem highlight being an assistant for the 2008 premiership as one.