Thanks for all the comments. I remain puzzled as my brain does not accept what my ears have heard. I can't eliminate the possibility that the presentation was somehow rigged, nor that it was a placebo effect. Just trying to find a plausible theory supporting this experience.
You don't need a rack to control vibrations - the rack's mostly for aesthetics. I've done well with just books n top of my ancient CDP, but later I got a bit of marble tile and glued a rubber sheet on one side. Same effect, more compact. With car audio CDPs the problem despite their working fine there is there's no way to test it on a rack in a moving car, but of course that would slide off, so basically assume that the mounting of the CDP chassis onto the dashboard plus some additional hardware inside between the chassis and transport helps. Plus the smaller surface area of the chassis of a car CDP vs all that space on a home audio player that can get all those vibrations.
Two things I did to eliminate their impact : first, I used headphones. I couldn't tell any difference with the same CDP whether it was loaded with weight or not. Second, I went solid-state (iPad+CCK) unintentionally when I got one as a gift, but no matter how effective that is in theory in all honesty I can't hear any difference between my laptop with a platter HDD vs the tablet (I can't really compare them to the CDP though since I was using them via USB), but then again, the HDD has a buffer.
In related news, I've seen worse voodoo from some audiopiles - like that guy on YouTube who wrapped all his gear in ERS Paper, but then he has his amp and preamp on a Spider Rack while his Cary CDP or transport was on his Ikea table.