If we're going to keep bringing the manga up and ignoring anything from the anime that wasn't in or conflicts with the the manga why don't we just move this all over to the manga thread?

In the anime (which we're supposed to be talking about) the flames do engulf the zombie-kage's but don't seem to do anything to them, then a few seconds later, while surrounded by flames Nidaime does the Water Wall technique and blocks further flames from reaching them. But as you mentioned, they were zombies, so it doesn't really matter that they couldn't be harmed by them.

In the anime (which we're supposed to be talking about) bodies often slam through solid rock without taking significant or incapacitating damage themselves, these are facts of the anime regardless of how things played out in the manga.