You are most likely right. Since I just realized the only similar bug reports relating to Chomp are from Tyrael and Uther kills, it is probably some kind of an error relating to their unique death states.
(Tyrael does not die immediately but becomes a spirit that explodes after a while, Uther becomes a spirit that heals teammates, Leoric becomes a spirit that roams the map)
So it is very likely it is a bug or error, relating to how these heroes have unique death mechanisms and don’t trigger a death for Chomp.
(If the wording meant only instant Chomp kills count for stacks, then it would apply to all heroes, not just Leoric, Tyrael and Uther).