The Mill obviously doesn't spell this out in their press release. But this is all about native advertising.
I think they use CG cars instead of real ones for the same reason everything is CG: cost. It's cheaper to just rent one of these from a local film studio than sending out a small fleet of actual cars to film in whatever salt flat they are using this time. It also gives them more control of the look and design of the car. Did you know that 75% of the items in an IKEA catalog are CGI?
Also, I wouldn't be surprised (given the tight release schedule of some car models and the secrecy around them) that this lets them film the advertising of a car concurrently with the final production stages. They don't have any cars coming off of the line to shoot with, so it's easier to rig up the wheelbase (which is established early in production) and get the marketing done concurrently.
I see you didn't watch the video where they showed the wheelbase, wheel sizes/design, width of the platform, as well as suspension tuning can all be adjusted easily.
Sure an SUV and a mini sedan won't but those are extremes.
Considering how many different car models use the same platform even when they are from different manufacturers since many of those are owned by the same parent company these days there is probably enough overlap to put slightly different care models over the same placeholder.