The clarification took place when spud3 reported leaving FL80 and the controller acknowledged, didn't correct. It gets more interesting if spud3's comms had failed after receiving the clearance!
DANGER, WILL ROBINSON!!!
Don't forget that, in the US at least, ATC is no longer liable for their mistakes, including failure to correct pilots for wrong readbacks. Per an FAA publication a year or 2 ago (I forget the formal title/doc number), pilots are still liable for certificate action if they "break the rules," even with ATC's implicit consent.