Why do, at least some of, these photos have such a strong HDR-grunge look? Is this what consumer film from the 70s looked like?

I noticed one of the captions mentioned it was a Kodachrome slide, and I found this website which has old photos and searched for Kodachrome slides (note I have ublock on, I think the website might look pretty terrible without it):

https://flashbak.com/?sfid=369046&_sf_s=Kodachrome%20Slide

A lot of them have the same vibe color grading-wise, I think it's a kodachrome look.

IIRC, Fuji came out with Fujichrome, which was a super saturated color palette, and a lot of photographers stated liking it and using it. So, Kodak had to respond with their own super saturated color film, which they called Kodachrome.

Kodak had been known for having super accurate colors in their film (outside of the whole issue around not handling well for darker colored skin), but they were losing a lot of business to the flashier Fujichrome, hence the need to respond with their own version.

On top of that, colors shift in old film, so what may have been a more natural palette may become more garish over time, although I would normally expect the reverse to be the case.