If it's a year after release, I think that's fine. They all saw an opportunity to make money, it sold far better on switch than the other platforms AFAIK, and now they have a racing tie in with a star fox racer being released this fall. That would make sense to me, and I really didn't even believe this rumor was true or false until the starlink footage was shown. Because of it's instant failure, it isn't the type of game that would see this kind of almost spin-off support, it at best would see a small expansion, but new modes should be out of the question for a 6 month old game that bargain binned.
To me, it only makes sense if star fox GP is real, but it's not like your logic is unsound, I know what you are saying happens, and I know false information is created and given to insiders to leak, but this seems very different than those things, and Retro's title should be done. I mean we've seen ex retro employees say the game should be done soon, and Nintendo has been trying to figure out what to do with Star Fox and F Zero for a long time, it wouldn't surprise me at all if Retro had worked on something this past 5 years among those IPs.
Again we don't even know if this is a new mode. It could just be a side mission where you need to race a certain character. And it could have been in development long before they knew the game would bomb, in fact it probably was. I don't think this new mode is evidence that Starfox GP does exist... that line of thinking just doesn't make much sense to me.
As far as Retro being done, yeah it's likely they're done or almost done a game. I hope we do see it at this E3 because I can't imagine them revealing it any later, due to fans' expectations going to MP4 after that like you mentioned earlier. But just because they (hopefully) have a game done and ready doesn't mean it's this particular game that has been rumored. It might be something entirely different.