Any question answered...

Proposes it based on what?

I have no idea; maybe data, maybe bullshit.

I read it (the above post) as being proposed on the basis of the average couple height being 5'8'', my point being that that in itself means nothing. Obviously.

It would be interesting if the couple aggregate height were more concentrated about the mean than individual person height, since that would suggest that taller than average people preferentially select shorter than average partners (and, of course, vice versa). I think the theory, albeit poorly expressed, does propose that this is the case. As expressed, I think it proposes that couple aggregate height is always 11'4", which is demonstrably false, but I read it as implying that there is a statistically significant narrowing of the aggregate height bell curve compared with the individual height bell curve.