It is important to separate the notions of validity (“does this chain satisfy all validity rules”) and being active (“among all valid chains, does it have the most work?”).
Let’s say the two branches of the chain are A (created by Bitcoin miners) and B (created by BIP110 miners). A has ~10x more work than B, but over time, after multiple retargetting, will have equal number of blocks.
To BIP110-enforcing nodes, only B is valid, so their active chain is necessary B. They do not even “see” the A chain.
To Bitcoin nodes, both A and B are valid. However, regardless of the number of blocks in each, or difficulty adjustment, the A chain has clearly more proof-of-work than B (which is compuated roughly as sum of the difficulties across all its blocks). Thus, Bitcoin nodes will never consider B to be the active chain, because A has just far more work.
Hard forks are unrelated; those are changes to consensus rules that are incompatible in both directions. That is not the case here, the BIP110 chain is valid to Bitcoin nodes, just inferior in proof-of-work, and thus irrelevant.
