It’s a difficult one this because although you can argue that theoretical doctrines of God are not essential for salvation (which would be correct), you have to understand that for any particular group/denomination, there are going to be doctrines that they find jarring to hear someone deny. If a Trinitarian believes that Jesus is God, as well as the Father and the Spirit, then to hear someone not only deny that but actually affirm other basic doctrines of Jesus’ life is inevitably going to be difficult to hear. It would probably be less ‘harsh’ to the heart to hear someone deny that Jesus even existed than it would be to say he existed, died and was raised from the dead and yet was not God. It’s only natural for Trinitarians to react strongly to that, especially if they believe that the very word Christian requires at least a tacit belief in the divinity of Christ; that would also lead naturally to consequences on what they thought the gospel was - I mean, if we can all agree on something about the basis of the gospel, it’s surely that Who God is is THE key part of it. Any fundamental disagreement about that (and the argument over whether Christ is part of the Godhead is definitely something of fundamental importance) will always lead to strong opinions either way.
I’d also add that Parry’s statement is not strictly about Unitarians themselves - it’s about Unitarianism. If you believe that Trinitarianism is a fundamental and clear doctrine “of the gospel and the biblical revelation of God”, then I think it’s perfectly understandable that you see Unitarianism as a betrayal of that. That’s not to say that Unitarians cannot be saved or are being dishonest or immoral in their following of God or that a denial of the Trinity shows hatred towards God or anything like that. It’s just to say that an intellectual belief of theirs is a false portrayal of the reality of God (or at least how Trinitarians see God).
However:
That statement has basically undermined everything you’ve said in your OP. It’s understandable you’ve got worked up about it but you cannot possibly complain about some Trinitarians’ attitude towards a group like the Unitarians and then add in a pathetic, childish sentence like that. Accusations of intellectual dishonesty and hypocrisy don’t go down well if you’re showing exactly the same attitude. We’re all in a position of trying to understand Who God is and statements like that really don’t help.