I don't feel like this is true at all. While the health regeneration certainly is a similar mechanic, the champions are really very distinct. For example, when grouping Mundo's cleavers are much more useful in a poke composition, where as Zac is far more useful diving in to the back line.
Mundo is still very viable in the jungle as well, where as Zac is much better as a top lane pick, as he typically needs more farm.
Mundo is a heavy sustain jungler, provides poke and tankiness.
Zac is a heavy sustain laner, can work in the jungle, and is primarily used to dive the back line.