I very much doubt Bamco would ignore an installed playerbase of over 110 million PS4s. They only showed the PS5 logo so far because it was a Sony sponsored event.
You'll notice that there was no PC logo, either. But the game is 100% coming to PC.
It'd be cool if they didn't have to develop for 10 year old hardware, though.
It only says PS5 because it was a Sony event, but Bamco themselves confirmed, in the actual event, that it will be for PS5/Xbox Series and PC only, and checked again, even on Tekken 8's Twitter page it says PS5/Xbox Series/PC only.
To be fair, Harada on Twitter says he knows a lot of people that do not own a PS5 and jokes that even the Sony people were suprised at the news, i guess Bamco expects that when the game comes out, more people have gotten their PS5's and abandoned the PS4/Xbox One.
I wasn't expecting this, but i guess developers must stop going for cross gen games at some point.