The fact that verisign (and various other registrars of .com) have other unrelated clients with other domain names who may be in Texas is unrelated.
Irrelevant. Texas has jurisdiction over anyone doing business in Texas, and may enforce all judgments, writs, and other decrees using third-parties who also do business in Texas (Verisign decidedly does) and typically throughout the United States.
The law isn't an autistic computer system; this isn't about "fairness." This is about "what can be enforced." This controversy is more of a political and diplomatic issue than a legal issue.
I'm sure a creative and bigoted Ugandan government lawyer could argue that our client is "doing business" in Uganda by publishing LGBTQ content that is accessible to internet users there.
Does somebody shitposting on a php bulletin board hosted by one of our colo customers insulting the king of Thailand mean that we're subject to judicial sanction in Bangkok? Should I expect to be extradited to stand trial in Kampala? It makes just as much sense.
Does Verisign lose anything of value by being forced to stop doing business in those countries?
Texas has the power of being a member state of the United States. The Constitution grants Texas the ability to enforce its judgments throughout the lands and upon its own businesses.