Biological determinism is not necessarily full determinism. External circumstances could have been other than they were, and then the biologically determined decision could have changed. Plus, for every deterministic model, the same arguments would also support a world that also includes pure randomness in addition to the deterministic laws.

Still, the author is making a deterministic leap by stating that they wouldn't have been X if Y.

I'm not making a deterministic leap I'm leaving out vast amounts of information from my story that are way way more personal. Of course there are more things that I know that don't make this a deterministic leap but I don't care to share them.