The stronger “Use it or lose it” rules only applies to LPONs; 1341 is high-power.
In this case, the broadcast licence for 3GL was never fully given up - more correctly, a new 3GL broadcast licence was created when the “old” 3GL converted to FM as K-Rock, as the 3CAT licence of the latter kept the 1930 start date. (Similar situation to some of the other Victorian FM conversions, where the AM station was reborn as a new licence afterward.)
This might’ve been fine at the time - indeed, briefly it could have been run as commercial - as I’m not sure they had quite combined with Bay FM at this point (can stand corrected on which absorbed the other, this post from last year suggested Grant got Bay FM a little before Hoyts sold K-Rock to them, but the time difference would’ve been minimal so, details 
)
That’s only useful paired with an apparatus licence - and the current HPON licence was only activated in July, so even if it was low-powered, I think that’d be too early for use it/lose it to apply.
Given the various few services in community space where “seniors” is seen as a valid community (Five-O Plus, Capital Community Radio in Perth, etc), I suspect 50s/60s would probably be arguable as narrowcast - it’s a bit of an abandoned format in commercial space after all; that age is more or less in “Silver Memories” territory now.