Well, the only ways I could find to clear the purgeable data were 1) creating a bunch of random data files in terminal until the drive was critically full (I didn't do that) or 2) download CleanMyMacX and run the maintenance setting.
First, I would generally NOT recommend downloading something like CleanMyMacX and I was very skeptical. I can't wait to get it off my system and I am not paying $39 per year to use it. But it did, in free trial form, delete my purgeable files, so I'll give them that.