This is absolutely not the case in my experience. Very strict day-of-departure only policy there, and not nice about it.
WOW! That's weird. Were you connecting? Maybe this what matters for them. The first time I went to KLM lounge was just by chance (it's 20 m walk from Mercure). I didn't expected to be admitted. They just scanned my bp for next day flight and that was it. The next time (December 16) the experience was similar.