"They were iterating and releasing tablets even after the iPad was released. They just didn't make the leap that Apple did to create a tablet that appealed to the masses."
I think this is a succinct refutation of the Tablet PC being a counterexample to something "ahead of their time". The lack of success of the Tablet PC wasn't because it was "too early for the technology", but it's because Microsoft and HP were developing the wrong device for the wrong market.
That said, I do tend to agree with your premise that you certainly can be "ahead of your time", and that being first to market does not always correlate with being the best to the market.