It's unlikely that WoS would have sued had KD not decided to call him a homophobe in the Scottish Parliament and then in a big column in a daily newspaper.
However, WoS has supported gay rights vocally and persistently on his website (there are supportive articles about the same-sex marriage campaign going back to 2012 on his blog), and privately without talking about this as was revealed when gay people told their stories of his support for them.
The tweet is a tasteless, mean-spirited joke. What it isn't is homophobic - quite the opposite as he expresses a wish that a gay person had embraced their homosexuality earlier than that person did in reality.
I would guess that the reason the defence is going heavy on the transphobic stuff is because it is now widely assumed that if you oppose the T you oppose the LGB and because accusing someone as transphobic is almost a surefire way now to ruin that person's reputation (and more).
Whatever the outcome, this is an important case for women's rights campaigners in this debate, as WoS supports equal rights for trans people but opposes GRA reform, self-id, compelled speech, that sex is a construct and most importantly opposes the erosion of women's sex-based rights. He does this in uncompromising fashion, and ever ready with a swearword if he feels like it (he feels like it a lot).
If the defence convinces the court that WoS is transphobic, homophobia is an easier sell for them. It's a strategy that would seem to be promising right now, as many GC people have been branded as transphobic for far milder statements, with much success.
And it's interesting for us because he explicitly rejects TWAW with an absolute refusal that males who identify as trans can ever become women.
But freedom of expression actually includes the right to offend, so the rude tweets alone probably wouldn't do the trick. Never know these days though...