Those emoticons seem to have been in many ways better than :-) and its relatives, but it sounds like they 1) relied on details of the platform, and (related) 2) never caught on more broadly. So I think it's still reasonable to celebrate the invention of :-) (while maybe imagining how much richer text conversations might have been if the PLATO text-display system had become ubiquitous).

I was definitely using :-) on Plato in 1979. I'd be surprised if Plato was the first use, though.