Touchless control is different from the assist app driving mode, unfortunately. However, I'd think you could try saying "ok Google now" and see if pin unlock works; and give commands that way, rather than accepting the offer to reply to text messages from the assist app.

I do believe there was a time when this worked, though, shortly after the phone was released.

 

They're two different apps/services. You can turn one off and still have the other work.

I never use touchless control but I recall reading that speak pin to unlock is broken. I could be wrong about that, but too bad if it is.