Given this fork still boasts a 2-clause BSD license, the corporate nginx can still make the effort to backport patches. It's certainly harder than requiring a single converged development branch, but how closely they track Maxim's work is ultimately up to them.

If nginx continues to receive more attention from security researchers, I imagine Maxim will have good reasons to backport fixes the other way too, or at least benefit from the same disclosures even if he does prefer to write his own patches as things do diverge.

Though history also shows that hostile forks rarely survive 6 months. They either get merged if they had enough marginal value, or abandoned outright if they didn't. Time will tell.