You are correct. Looked it up, and Jeans are sub-category of Pants. Though, I live in the US and have never had someone refer to jeans as pants. It seems a technical definition that I've never seen used that way. I know arguing with HR they did not see it that way.
Perhaps HR really meant 'slacks'. as in Dressy Pants.
Really, I had to look it up. I had always thought of 'pants' as 'dress pants'. So to have such a broad category of 'pants' seemed like an older technical definition I've never seen used commonly.
But, if you saw in other posts. For the Chess rules. There was another section of the rules that specified 'no jeans'. So for the current controversy, it didn't specifically hinge on this definition of 'pants'.