Sony's PS5 UI video probably wasn't being used to tease a new Killzone game, despite what some fans might believe. A recent post from Reddit user Cryptostormz noted that Killzone appeared to be front-and-centre of yesterday's video, but it's looking unlikely that that's going to transition into a full-blown game.
When Sony began its UI video, PlayStation senior director of global content communications, Sid Shuman, noted that he was booting the console up from rest mode, and had already created an account. As Crytostormz points out, not only did that account use a Killzone avatar, but the character the account is named after - Kenzo Hakan - is pretty obscure, his narrative hidden away in an audio log about leaking hidden information. Sony could easily have used the likes of Aloy, Kratos, or even Peter Parker in this video, so the double Killzone reference had fans of the series excited.
Sadly, however, it looks like it's probably not to be. As pointed out elsewhere on Reddit, Sony has been using that Scotty Kenzo name and avatar since at least 2017, when it was discussing the PS4's v5.0 update. It might be notable that it's been carried over to the PS5, but it's unlikely to have been an out-and-out teaser.
Of course, it's been a while since the last Killzone game - Shadow Fall was a launch game for the PS4 back in 2013 - which could suggest that something is in the works, but with no sign of the series in the PS5 launch games, if we do ever see another entry, it's likely to still be some time away.
Perhaps one day we'll see Killzone on a list of upcoming PS5 games, but for now you'll just have to stick with that we've got coming already.