I actually feel a little sorry for this HR guy. On one hand yes he's obviously been professionally incurious and has got the law wrong - which as an HR person is literally his job.
But at the same time, he is probably the most targeted and indoctrinated - sorry, I mean "highly trained" - in trans rights thinking within the organisation. Imagine you are a nice guy working in HR at a male dominated engineering company, came up through the ranks, maybe a bit older and worried you're a bit out of touch and there's a mandate from on high to improve reach to youngsters and look good on the old LGBT stuff. I can imagine he was a sitting duck for all the trans activist misinformation coming out of organisations like Make UK, who are effectively pushing Stonewall law out into their members via policy and legal advice.
I bet he went through hours of training, conference panels, bumf that he was reading and stuff he was hearing from his peers at other companies on his Make UK committee lapping it all up and nodding along without ever even once thinking about how it affected the women in his workplace. Or if he did think about it, he might have then thought - well the women might not like it but they need to get with the times, like me! I'm hip and with it, and down with the kids! And anyway there's a secret squirrel toilet they can use, and I have to support the poor trans people in the office first, like I'm being told I have to by everyone else in my industry. I think that's also why he kept bleating about there never having been any complaints from women, because that was proof in his mind that he was doing the right thing.
I hope the penny has now well and truly dropped that they fucked up. But he will quite rightly be able to say that he was advised by every expert he consulted, both legal and HR, that this was what he was supposed to do. Add to that that every other organisation he knew about was doing the same and his position, while wrong, is at least understandable.