For sufficiently loose definitions of "working tablet", you are of course right. Like those Microsoft thingies that everyone stayed away from in droves. Or Apple's own Newton.
So why was the iPad the first massively popular one? Because Apple produced one that was actually useful rather than started people cursing in the first few minutes. That took them years of experimentation and polishing; there is a reason there had been rumours about an Apple tablet for years before it was actually introduced.
It's easy to think up the concept of a tablet, and even to build some vaguely functional prototypes, but until Apple came up with hardware that was light and sturdy enough to be practical, and software that made the limited environment useful, tablets were something you only used if you absolutely had to. And Apple had nobody to `sponge off', this was Apple's effort only.
As far as I'm concerned this effort deserves the word 'invented', but by all means go for `reinvented' if it makes you feel better.