"I'd like to think NO professional (or even private) pilot would ever interpret that clearence as "descend BY 5000ft". THAT is not a standard clearence."
As an RT Safety Advisor, I can assure you that "descend 5000ft" is misconstrued by crews more times than you can imagine and even recently, has been responsible for multiple events of level busts and unfortunately, in some cases, CFIT incidents as well.
Reports of stuff like this ends up on my desk on a regular basis.
"ATC never ever issue instructions/clearances with relative altitudes."
Not true. It happens. A lot! Not necessarily where you are but if you travel round a bit, what you see and hear can be enlightening!