I know a bit French and atmospheric pressure is pression atmosphèrique (et non pression d'admission comme on peut lire en bas d'echelle) - even for engineers. Perhaps with some poetic freedom they would have written pression ambiante. It is however written pression d'admission and this is a technical term. I checked and the most suitable translation are inlet pressure or manifold pressure.

See translation here:
http://dictionary.reverso.net/french...'admission.

I have no clue what exactly they address here as pression d'admission and of course this is open to debate. I am however quite sure that they don't mean atmospheric pressure. Perhaps a Frenchman could tell if one is around?

EDIT: My calculator sais that 980 mmHg = 1.289 atm (=ata?)
and 990mmHg are 1.303 atm

So basically the French achieved 494 kph at 600 m with rpm 2400 and 1.303 atm pression d'admission
Extrapolating pessimistically to 0m they got 478 kph with rpm 2400 and 1.289 atm pression d'admission