A bit condescending?
I think he simply means because people always talk about Lgbt which groups lesbian gay and bi with trans.
If the assumption alluded to by the poster is common enough and an issue important enough to challenge then I would like to see a few examples of this assumption being made. In news stories or on forum or reddit posts would suffice. I've never seen or heard about it and it sounds like a malicious fiction about how the trans community banner is swallowing up gay and lesbian voices/spaces/resources. A fiction being used as a wedge to drive the t from the lgb.
However they may have misspoke and your interpretation may be what they meant.
Well then it should be immediately obvious to anyone who has a cursory knowledge of history, culture or the present, why such groupings exist, in the same way that it is clear why lesbians and gays have been traditionally grouped politically. One could point out that gay men have different genitals and biology to lesbian women and whine about people assuming that gays are part of the lesbian community. You can cut this up in as many ways and as many times as you like to divide and estrange those of shared political aims - a common tactic to alienate and suppress.
Why did Frederick Douglass lecture on rights and freedoms in Ireland? He was black and fighting slavery surely he didn't belong there.
Why did Desmond Tutu speak out in support of Palestinians? He was a Christian that campaigned against Apartheid South Africa, surely he was out of his element.
Why did Huey Newton speak on gay and lesbian liberation when his lane was black power?
Why did Fred Hampton align with white and native groups under a socialist banner, when he was a black activist.
Angela Davis speaks in support of trans women but she is a feminist. How is that allowed?
Allyship, solidarity, mutual aims, civil rights.
If it was ignorance then there was enough in my reply to help them educate themselves, and if they were being a sly fecking weasel then they deserve all the condescension in the world.