It's also possible I'm misremembering, but I'm about 90% certain since I remember I would do a pass through my inbox on my phone because of how quick and easy it was to sort this way.
Aha, found it[1]. This jogs my memory enough. I changed my "trash" folder to be my archive folder and could just swipe (or as the manual says "Throw the message off the side of the screen").
1: https://manualsdump.com/en/manuals/palm-pre/221591/213