The standard definition of strength, which the post cleverly avoided ever stating, is "the ability to produce force against external resistance" or some variant thereof.

Does this definition resolve the problem posed by the OP, that competence in one of various different specific activities requiring strength doesn't imply competence in the others? That is, after all, the basis on which IQ tests are attacked - competence on Raven's progressive matrices doesn't imply competence at the Piano. If we would answer their objection by saying that intelligence is the general capacity to solve problems, have we shed any light on what ties these capacities together?

If by "obvious" you mean "the sort of thing you might guess from first principles"

That's what I interpreted James Miller to mean, at least roughly.

the muscle-strength relationship is obvious in another sense: in actual data, it will leap out at you as a very large factor. For example, 97% of variance in strength between sexes is accounted for by muscle mass, and one of the strongest predictors of performance in powerlifters is muscle mass per unit height.

Seems to me to be merely a difference of degree. While not "leaping out", brain-mass and intelligence do seem to correlate non-trivially (at least when cranial volume is measured via MRI):

Among humans, in 28 samples using brain imaging techniques, the mean brain size/GMA correlation is 0.40 (N = 1,389; p < 10−10); in 59 samples using external head size measures it is 0.20 (N = 63,405; p < 10−10). In 6 samples using the method of correlated vectors to distill g, the general factor of mental ability, the mean r is 0.63.

http://www.ncbi.nlm.nih.gov/pmc/articles/PMC2668913/