I'm surprised by this (unless it was a while back). My observation is that whilst I've seen airport workers/EVA AMEX platinums etc. get in, I've not seen "normal C class" pax gain access.

Prior to the refurbishment of "the Diamond" (originally it was the First Class lounge), "lounge upgrades" were common and as Gold I was often given the Garden.

I was Gold for a while after the Diamond lounge opened. In Elite/Economy I was always put into the Business lounge and in PL the Diamond lounge. Never the Garden until I got the Diamond card.