Thank you very much, very interesting info.

Possibly relevant:

Whilst the French had not seen any technical information on the .280" round they were of the opinion that the stopping power of the ADE ammunition was likely to be inadequate. They therefore believed that NATO ought to adopt the .30" calibre and made it clear that France would support the US in questions relating to small arms.90

90 Report on the Small Arms Conference held in the USA on 2nd and 3rd August 1951, CAB 21/3465,
NA.

 - Ford (2008) p.191

Of course, it is the opinion of entirely different people from those who chose 7.5mm in the first place given the time lapse. You could also interpret "stopping power" in terms of energy rather than caliber (though harder to square with "had not seen any technical information").

To switch the topic to the familiar .280 British saga, rereading Ford was interesting. While he misses the trajectory and special purpose bullet concerns that most other versions emphasise (and seems to be inaccurate in his claims about retained energy), he gives a lot of references.

One interesting titbit is that the goal of a select fire rifle (thus presumbably the Ideal Caliber Panel and the .270) was itself a compromise to appease British marksmanship enthusiasts. The operational researchers just wanted to give everyone Stens, which were considered as good as a Bren up to 300 yards (pp.117-120)! 

Also Churchill couldn't resist chiming in:

When it was pointed out that the British infantryman had been expected to use .303" ammunition for 50 years and that it was time for it to be replaced, 'Mr Churchill replied, with a smile, that we had used the long bow for very much longer than 50 years'. (p.194)

'When I was at Omdurman I rode with a sabre in one hand and a revolver in the other' to which
Slim retorted, 'Not much standardisation there Prime Minister'. (p.197)

And half a century before the (British) NRA opposed the SMLE because it wasn't the ideal match rifle (p.99). 

The ".30" refered in the text is the .30-06, and France was OK to use this round for rifles and MGs, should the US decided to push this round into NATO standardisation process.

The ".30" like the T-65 was less well received, and France transition from 7.5x54 mm to 7.62x51 mm was a very slow process.

Why did they like the .30-06 better?

Look at the date (1951).

At this time, the T65 cartridge was still probably loaded with the 137 gr T-104 ball and the 136 gr T-93 AP used for the comparative tests performed in february - march 1950.

Those loads were balanced for rifle application, but did not provide the long-range performance of the German 7.9 mm s.S or the US .30-06 M1 cartridges when fired from a GPMG, which was the "hotest trend" for western armies after WWII.

The case volume and cartridge length of the T-65 were probably considered too small to fit a slender 180 gr class bullet and give it the required MV.