If you look at development times for TMY at 400 (box speed) and at 800, pushed 1 stop, the development times in XTOL are identical, the 800 speed has less shadow detail recorded.
Experts chime in please, but, the zone system/under/over development, wasn't this much easier to do with the older films. Ektapan, Super XX etc.???
Mike
TMY metered at 800 isn't pushed, it just has less exposure than if it is metered at 400.
You don't "push" film at the exposure stage, you "push" it at the development stage.
The reason that Kodak doesn't recommend an increase in development because the increase in development always involves a compromise. While the increase in development improves the contrast in an otherwise "exposed less than optimum" negative, it usually decreases the quality of how the highlight details are rendered.
In Kodak's judgment, the shadow detail on the 800 metered film is of acceptable quality whether or not the development is changed, and the contrast improvement one gains from increasing the development of the 800 metered film is more than offset by the reduction in quality of the highlight detail. So Kodak doesn't recommend more development.
For clarity, and even if it is redundant, increasing the development makes essentially no difference to the ability of the film to give good shadow detail in under-exposed films.
And yes, with respect to new vs. older films, the newer films like the T-Max films tend to offer longer straight sections in their characteristic curves, so they encourage the photographer to use printing controls instead of development controls to achieve similar results.