Every Ph.D I've met has been a very hard worker, on par with the people I worked with in the military.

I never seen PhD as a program that is attempting to teach people write. There is absolutely no reason to expect someone with PhD to write better then other people.

If anything, a PhD program teaches you how to write faster while maintaining an acceptable quality: Journal papers, conference presentations, and funding agency reports are all long-form writing projects that come up quite frequently and distract from the research (the fun part). Most of the value from this writing comes from it being accepted; improving quality beyond this level has rapidly diminshing returns.

My writing definitely improved tenfold.

Weirdly I have personally never considered PhD to be indicator of being particularly smart. Probably above median, but not special. On other hand it sure requires lot of work to be done. Or at least time spend.

That tracks. I’d add that Ph.D. laureates tends to be people who like to pursue knowledge in depth. I think it’s this, combined with the work ethic and discipline, that make them excel in certain fields.

"Hard working" and "smart" are very different things.

One could almost say that not being smart requires you to be harder working. (At least that's what I'm telling myself to keep working hard.)