The first one is true. The second one isn't.
Big Rigs was simply an unfinished, un-polished version of Stellar Stone's real project, Midnight Race Club: Supercharged.
Compare.
Midnight Race Club: Supercharged is the sequel to Big Rigs. Stellar Stone was working on it full-time, but released Big Rigs early while the game was still underway. Because of this, most of the game remains unfinished. Levels were cut, sound was ignored, AI was completely skipped over, and sloppy translations for an English market resulted in You're Winner.
The game was supposed to be full-featured. Not only would there be a Single Race mode (for doing one race), but also a variation on Mario Kart's Grand Prix called Competition Mode.
Nightride, the fourth course, was unfinished when Big Rigs was released. In original releases of the game, the level simply crashes. In later, "fixed" releases, it was replaced with a mirror image of course 1. Nothing remains of this level.
When Midnight Race Club WAS released, however, it contained this course.
Yes. There were cars.
The game was originally supposed to feature 4 classes of 17 vehicles: sports cars, pickup trucks, big rigs, and motorcycles (as well as a Hummer which didn't make it into any category). All vehicles were completed, but for unknown reasons (possibly because a proper selection screen was never finished), only the big rigs were playable. All the vehicles' data files, however, were still included in the game.
Other resource files indicate that Stellar Stone had a lot riding on the game. They have extremely hi-rez, unused textures, bumpmaps, cubemaps, water, etc. None of that ever made it to Big Rigs.
On January 2004, Stellar Stone finally released Midnight Race Club: Supercharged in Russia, and it was a failure. The game looked exactly like Big Rigs. Some things were improved, however, such as opponent AI being added and the horrible driving physics (according to a review, now you can't go up hills at all). The game also removed the big rigs entirely, possibly so the game didn't seem too repetitive.
Unfortunately, everyone realized it was just an improved Big Rigs base, and hated it. We will most likely never see the game hit the USA.
Big Rigs is incredibly unfinished. There are unused sound effects lying around, unfinished levels, water textures, old and un-removed files, and even some .psd work files where they made various images.
Of course if you have a copy of Photoshop, that's the fun part.
(all this was discovered by me, except for images with a logo in the corner. Those screenshots were from a Russia website that reviewed Midnight Race Club: Supercharged.)