Garden Chambers had to payout £20k because of a tweet they made on their own accord (see the thread about it for details). This is nothing to do whatsoever with Stonewall.
Please stop misrepresenting this case, I'm going to correct it every time. It wasn't just one tweet they made.
Garden Court Chambers (at least try and make it look like you have any idea about it) were found to have discriminated on the grounds of Allison's gender critical feminist beliefs on two separate detriments, the comms team tweet you mention saying that Allison was under investigation, but also the disciplinary investigation they carried out which was in direct response to the Stonewall complaint, among a couple of others.
So yes it was to do with Stonewall, it just wasn't found that GCC's discriminatory investigation was caused by Stonewall's complaint.
They were also separately found to have victimised her by carrying out the discriminatory investigation due to a protected act (her tweet about Stonewall)