The claim about hydrogen engines having to emit NOx is just a myth. When hydrogen burns at a phi below 0.5, the amount of NOx emitted drops to nearly zero.
In other words, a hydrogen engine can be truly NOx-free. As a result, H₂-ICE are fully possible and are genuinely zero emissions engine.
In reality, this is just a vicious lie made up by BEV fanboys, trying to demean a perfectly good idea.
https://www1.eere.energy.gov/hydrogenandfuelcells/tech_validation/pdfs/fcm03r0.pdf