I think this post illustrates how completely TRAs misunderstood and misused the 'Overton' window.
Firstly, the Overton window is a libertarian idea - it posits that optimal policy solutions often fall outside the parameters of any given debate; it's the job of think tanks to expand the parameters of the debate by proposing unorthodox positions, those positions might rarely be practical in themselves but they stimulate discussion that does result in the formulation of creative solutions of policy problems that would have been intractable if the discussion had continued along the same, constrained course.
As others have mentioned, TRA tried to use the concept as a form of mind control, of using the content moderation mechanisms of social media platforms as a way to set hard limits on the Overton window (by deleted posts and banning users) and social shaming as a way to punish thought crimes against GI, including depriving the holders of those beliefs of employment. It's a grotesque, dark misuse of the concept.
What's interesting about the blog post is that most of what he talks about is simply to reiterating trans talking points that are already well inside of the Overton window: inclusion, pronouns, kindness, personal stories, etc. I suppose you could argue that what he's suggesting is a rearguard action to keep the Overton window from slipping even further to the GC side. (Not as though ideas seem to be going away any time soon!) Notably, he's not offering anything at all that expands the window. Instead he's advocating for TRAs to regroup and rally around their old tactic of trying to constrain the window by making certain topics off limits:
"1. Name the regression.
Don’t pretend the current climate is a balanced debate. It’s a backlash. Say so. Make space in your communications to talk openly about the coordinated erosion of trans rights.
...
"4. Challenge false balance.
Don’t platform “both sides” on identity. Inclusion is not a debate club. It’s a commitment to equity. Don’t give hate airtime under the guise of free speech."
To stretch the concept a bit further, since we're talking about what can be discussed, it's apparently still not possible for TRAs like the Lockwood to acknowledge the 'controversy' that some people think TW are men and that therefor TW's desires to use single sex spaces and services compete with women's needs to exclude men from them. The fact that that argument cannot be acknowledged in order to be refuted, suggests that TRAs also have an internal Overton window and that part of their current distress results from not being able to participate in the public discourse, because this misalignment between public and private windows means that straying too far outside of the internal window, even to refute assertions, has too much of an emotional cost.