I was looking at these screenshots again, and got to thinking... Man, even besides the resolution thing, and despite the dev plainly stating "This was for Nintendo DS"... This does not look like a DS game. Just look at any other LEGO DS game. There's no comparison.
It does, however, look more like a Wii game. And the screenshots are 640x456... I know little about the Wii, but a quick google search revealed this: https://gbatemp.net/threads/vwii-mode-overscan-issue.340460/#post-4515450
So. Perhaps a DS version existed too, but this stuff we've seen is almost certainly of a Wii game.
Unfortunately, when the developer who posted these renders and screenshots was asked about them some years back, he didn't answer, and removed both the Blogspot and Behance posts. The contents were archived, so nothing was lost, but it's clear he doesn't want to be contacted about it.
Honestly though, I'm not interested in trying to hunt down more info on it. Looking at the screenshots we have, what Firebrand's other games are like, and comparing that to the original LEGO Racers games... I just can't see it following in their footsteps.
Also, I'm not sure the Firebrand guy who went on to work on LU at NetDevil was involved in this game. Firebrand has four studios (Glasgow, Scotland, Florida, and Louisiana), and the person who posted the screenshots was in the UK (presumably the Glasgow studio). The person who went on to NetDevil was in Florida. And as far as I know, there's no official source anywhere saying he worked on LEGO Racers The Video Game, it was just assumed.