Fungi definitely play a big role in our modern defense systems (though saying we're "rooted in fungi" naturally reads like your suggesting we evolved from fungi since evolution is normally discussed in terms of roots, branches and stems. Perhaps just a language issue.

You specifically said "everything else died", though, which isn't at all ambiguous, but is entirely and clearly untrue.

Crocodiles are in largely the same shape now as they were before the asteroid. Lots of other organisms survived and continued evolving according to the prevailing conditions (which were largely a fungi-rich habitat, so yes, evolved to use fungi to their advantage... just like we also use bacteria and otherwise parasitic lifeforms to our advantage (think eyelash mites keeping you clean and the bacteria in your digestive system).

Symbiosis is a very normal part of evolution... Dinosaurs will have had symbiotic partners too.