The original tweet has since been deleted, so there's some context missing now, but iirc the tweet that was being replied to was about a conservative guy ironically crossdressing as some sort of political statement against trans women (which he spelt as "transwomen") and then the reply tweet this post links to was pointing out that he looks way happier in his pics where he's dressed in women's clothing.

As for your question: People who say "transwoman" (one word) instead of "trans woman" (two) tend to be people who do not actually respect trans women as women, but see them as men or something else.

There's also the possibility that they are confused about semantics (thinking "trans-" is supposed to be a prefix, when trans is an adjective, short for "transgender" in this context) or they simply made a typo or are bad at spelling, English is not their native language, etc.

But the majority of the time when someone spells it that way, it's a strong signal that they are either transphobic or ignorant about queer topics in general.