They couldn't have used it for fast air anyway, as far as I understand, due to the runway being so short. So they had the choice of either extending it, using it only for Pucara, C-130, Macchi and Turbomentor ops or not at all.
The pertinent fact is that they hadn't extended it prior to May 1, which they could have done had they really wanted to. From this I draw the conclusion that they didn't plan to use Stanley as a base for fast air. You may be right as to the reason why not.
I have read an interesting USMC analysis which points out that the deployment of the 'COIN aircraft' was questionable due to the high threat environment, but apparently their CAS skills were quite good, so possibly worth the risk.