I generally agree with your sentiment, but Twitter is a US company with most of their employees residing in the US (at least presumably?). So in this case it’s reasonable to assume they meant it’s a holiday in the US.
It might be a surprise to you, but people who don't live in the USA have no idea of what the holidays in the USA are!
As someone not from the US, it’s not a surprise at all. But if someone says a foreign company is making their employees work on a holiday weekend, I don’t fault them for not explaining what specific holiday it is.
Yup! And as somebody living in the United States, it is not so obvious how I would learn about Russian holidays, etc.
I would not be surprised if Twitter is substantially Indian or spread across the world.