The rumour has been running for ages and that's my point. Plus I haven't heard or seen that the firmware hold up for the z9 was due to the failing Z7 as it was only meant to be for the one model. Obviously another false rumour.
Lots of rumours fly around about lots of things, but I have only seen specs for a Z50II mentioned on Nikon Rumours in the last week or so, which means, for me, it's only recently become a rumour worth listening to. They have suggeste for some time that a Z50II is a likely next Nikon camera release, but I don't count that as rumour, just them making an educated guess.
Again, there may well have been rumours floating around about a Z9 firmware upgrade but the most recent rumour (on Nikon rumours) as far as I can remember, was for a multi-model firmware upgrade, with I think a suggested date period, which didn't happen, and they came back with the Z7 explanation.
But they are still just rumours, gathered from a range of sources, not announcements of events known to happen, so it's to be expected they will get things wrong,
But that isn't the same as just making things up, which I don't believe Nikon Rumours does,
Nikon Rumors is the only rumours site I look at, and there may well be others. I have occasionally been duped into clicking on a Youtube rumour type video, but these have always been obvious fiction, and quickly closed. I never click on them now.