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!
Yup! And as somebody living in the United States, it is not so obvious how I would learn about Russian holidays, etc.
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.
I would not be surprised if Twitter is substantially Indian or spread across the world.