I agree with Tomo's take on this. Red, you said you saw the fellow turn off... Well, that being the case, you knew the runway was clear. So go. The circuit traffic doesn't stop because one bloke is slow making, or forgets to make, a clearing call.
The 310's warning (for that is probably what it was intended as) was incorrect, because the traffic had cleared and you could see that. It is possible the 310 couldn't see what you could. A call you could have made after the 310's call along the lines of, "Yes he has," may have put the Cessna's mind at rest. But given the traffic you have described, I would have done what you did, and not cluttered the airwaves with the unnecessary chatter.