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.