^Spiner has claimed that some of the cases where it sounds like he used contractions were really just the result of talking quickly. But whatever the rationalization, the point is that there are far more instances where he did use contractions than there are instances where it was claimed to be beyond his ability. So I choose to go with the preponderance of evidence, especially since there's no logical explanation for why he'd be incapable
of doing it. The most reasonable interpretation is that he can do it but has to think about it, that it doesn't come automatically like it did for Lal. Probably what he was impressed by was not that she used a contraction at all, but that she did it without realizing, that it happened spontaneously rather than being a conscious affectation as it would be for Data.