How many of you know GitHub commits is designed based on Directed Acyclic Graph concept ?

Considering the core concept is treeish spanning, I would hope everyone. Not really GitHub though, that's a fundamental feature of git itself.

Based on my limited knowledge of got and data structures, I'd say it has something to do with trees and/or hashmaps

Cyclic commits wouldn't make much sense

I don't think, it is possible to make a usable VCS without having a graph of commits between code base states. Commits by their very nature are directed state transitions.
It is possible to build a VCS that allows for cyclic graphs (and detects duplicated codebase states to make the graph cyclic in that case). But it would complicate things for no benefit as having duplicated codebase states connected via commits is not desired and also unlikely to happen.