I think that GameXplain, Eurogamer and others said that they heard of it from a source even weeks before the information about Star fox grandpix went public, so they seem to back that rumor. It was also mentioned that the dk crew would be in the game, which would be really cool like
Diddy Kong
said, a throwback to diddy kong racing.
The only thing that's fishy to me is that I don't quite understand why they would work on star fox now, I mean the last star fox game all didn't sell well and critics weren't that good either. Whereas DK last outings were critically acclaimed and sold well (even tropical freeze sold almost 2mil copies on the failed wiiU and the port on switch sold almost 300k copies first week according to vgchartz, which is quite impressive and a better opening than back then).
But to be honest, it would be really cool if Nintendo themselves would be in charge for the next entry of the dk franchise, maybe we will get our "DK Odyssey" or "DK breath of the wild" from them!