We got a little more than prototype and concept. And even if it was as you said, would be the first time I remember seeing a next gen sucessor being show presented/confirmed this way. So it on a being nice to shadow it is acceptable to consider that an announcement of PS5. And saying that a name have been given on the case of PS5 is funny since there is probably 99:1 chance of it being PS5.
Was any of the technology used still in development? I'm fairly certain I read that SSD they were using was still in late stage development. That would make any model they're using still technically prototype. It wasn't the announcement of a PS5, it was just a showcase for some of the tech they intended on using. Yes, confirmation that a new console is coming. No, I don't find it to be a proper announcement.
Eeeeerrrrr until like 3-6 months before the console actually release it is still in development.
You can bet devs already have the intended specs (which can change just like PS4 had a double RAM that only first party knew because it was very near the launch of the console) and it isn't unusual that they would already have intended dev kits, but also not unusual that the final dev kit is delivered very near the launch as well. I remember case of it being finished after the HW was already out.
So even PS4 that was revealed in Feb and launched in Nov wasn't totally finished, from what I remember there wasn't even any game show in the event.
All together we can safely say that PS5 and nextbox were formally announced.