I find this defence utterly bizarre. In my employment contract, and I'm sure most other people's, there is a difference between what I do in my own time and what I say as part of my job. KM can say whatever KM likes in KM's own time, but KM had the weight of a big, influential, well funded organisation behind KM and support entourage and sought to cause a detriment to AB as part of KM's role in Stonewall. That's very different than whatever KM's views are down the pub.
The big problem here of course is that Stonewall has been behaving as if the law is as they want it to be - i.e. misogynistic and denying women their sex based rights. Sadly not as it is.
The problem is that KM's views were the same as Stonewall's views and as an institution the EA2010 applies to Stonewall and it's employees and their actions as part of their jobs, it doesn't apply to KM's individual beliefs down the pub.