I'm pretty sure all updates after the Final Fest are just balance patches.
On that topic, I find it funny how, back when version 5.1.0 was announced in November or whenever it was, many players mistook this for a new content update and overreacted to it a ton, saying stuff like 'Christmas came early' and 'IT LIVES', clearly missing the part where it said this was nothing more than a balance patch. Anyway, rant over, and this wasn't directed at anyone on here.
That said, I find it weird that, back when 5.1.0 was announced, they also said they were planning another balance patch for April, which was 4-5 months away. I'm no game developer, but I highly doubt it takes that long to create a mere balance patch. Still sceptical this update is going to be any more than that, though.
Edit: Forgot to say that, from what it seems, the 'SAVE OUR SALMONS' message was, unfortunately, likely meaningless. Would love to be proven wrong, though, as I really want Splatoon 3 to happen. I just don't want it to be another 'missing Marina' situation.