This is interesting, although "anarchy is the lack of hierarchy" is not quite correct. I would rather say that anarchy is lack of coercive hierarchies. Hierarchies do exist and will not cease to exist - if I want to learn about black holes, I'll ask Hawking, if I'm curious about chess I go to some chess master, etc. There is always some hierarchy, some are natural and voluntary, some are coercive in nature. The latter are present in capitalism and I see no reason why they would disappear in communism.
"Anarchist-communism" is not quite an oxymoron, it's just the two are words from different worlds. One means lack of coersion, the other is about specific social order. They are not mutually exclusive, but putting them together slightly bends their meaning...