Not sure I agree, but an alternative is hard to pin down. The problem is the dearth of photos of Eugene Martin's car, usually known as the BMW 328 special. The other one looks nothing like the one you have shown in your photo, yet ostensibly they are the same car, perhaps at different stages of development.The problem I see is that the unknown car has wheels much more typical of Simca engined cars such as Gordinis. The Martin photos show either steel wheels or larger wires and to me looks bigger.
The only other entered cars with a number in the 20s were Meteor - BMWs, a Simca and a Ferrari. It is clearly not a Ferrari - you can see a 166 circulating in the film. Jean Thepenier's Simca 1500 was almost certainly an ex-works Gordini that he bought that year. Did he have it rebodied? He was number 26.
Number 20 was Marcel Balsa's car, labelled a Meteor BMW. By this time his own Balsa car was powered by Veritas Meteor engine, but again it suffers from lack of photos except in its earlier form. However a later photo of this car fitted with its V8 engine has a distinctly similar nose to the unknown car.
At one point in the film you can see the back of the unknown car. It has a long tapering headrest fairing, and is very left hand drive - not a central seat. That would tend to rule out a Gordini, (but early Veritas Meteors were lhd.) It would have ruled out the Balsa except for the fact that I have a photo of a lhd car labelled Balsa which looks very like it.........but I suspect its accuracy. Ironically it could be Eugene Martins car!!!!
Still with me? I don't think it is any of these cars, I think it is more likely to be a late entry, small engined car as yet unknown.