I certainly wouldn't expect that to be the case. It's not necessarily true this game has been in development for a long time - Retro will have spent time building prototypes and pitching projects to Nintendo. Who knows how long it took before this game was green-lit? 

If they do have anything else in development, I'd expect it to be a smaller title - or to be something that's not going to see the light of day for a while. If Star Fox Grand Prix is real, it may use the Mario Kart 8 engine. Also important to remember Retro handled the DKC:TF port for Switch. They might also be remastering Prime Trilogy for Switch, who knows?

Ill leave it here what i commented on the Nintendomination thread:

"It still seems REALLY weird.I mean, it could turn great.Hell, if they implement combat into the racing a la Pursuit Force, it could turn into an excellent game.Still, its a rather.... bizarre idea.Specially since it suggest that retro was working on it while Star Fox Zero was still developing.I dont think that Nintendo would develop two games at once on an IP that has been failing recently.But then again, they were working on two metroids at once.

I dont know, this seems fake to me.But, I guess we will see."

Oh and by the way, it seems that Eurogamer is backing this rumor up.So its just not some random rumor anymore,

i actually think this sounds super cool. a star fox racing game with all the star fox gameplay included. blasting people in front of you to try to win would be fun.

It won't be the case this immediately began development after Retro wrapped up Tropical Freeze. They would have spent months - maybe even a year or two - testing prototypes and pitching projects to Nintendo. It's not like they have complete creative freedom. 

And if this did take four years, it could easily be down to a troubled development, a change in emphasis with Wii U tanking and Switch becoming the priority etc.

DKC TF would hardly make a dent on Retro workforce.Its almost a straightfoward port with one extra charachter and resolution bumped up.Im no expect at game developemtn, but it seems to me thats a job that a group of 5 or 10 guys can do in 3 to 6 months.

Sure, it wouldn't be resource intensive - but my points is if Retro are doing multiple projects, they're likely small. If this is Retro's next new game, you can bet the majority of the studio's resources will be invested in it. Just because it's mascot-driven and a racing game doesn't mean it won't have taken time, effort and manpower to develop.

If it is true, I am willing to be open minded about it. I think a lot of people are expecting this to be a Mario Kart clone with Star Fox skins, but the description by Stealth on Twitter seems to indicate it has more depth particularly in single player mode. If this game is indeed in development and it plays like Stealth describes it then I am down for it.

Mind you I am of the mind that Star Fox is a franchise with a lot of potential constrained by its genre. Its mainline gameplay has not been innovated on much since Star Fox 64. Thus, having the franchise done in a more relevant and accessible genre could keep it relevant if/until Nintendo focuses on the main genre of the game again. In many ways Retro Studios is doing something similar to what they did with Metroid Prime (taking a 2D Metroidvenia platformer and translating it into a 3D FPS with some of the key elements of the 2D games) and it is in line with what Rare did with Star Fox Adventures, Donkey Kong Country, Diddy Kong Racing, and DK64.

can you imagine diddy kong racing in space??

https://www.youtube.com/watch?v=Z8boRM6xRXg :OOO

Imagine if they did a Star Fox in the vein of Mass Effect only with more space battles. Now that would be awesome.

oh! So it's a sequel to one of the endings of star Fox Command lol!

I'm down for this though, Sounds cool.