That's as an advice for if you don't know what you want to do, as you pointed out History is a facilitating subject and with that you can do a History degree, a Politics degree, an International Relations degree, etc. Same with Maths, you can do Economics degree, Maths degree, PPE degree and so on. Whereas Politics is only going to be useful for a Politics/IR degree.
With that said, I'm doing History, Politics and Sociology and I'm looking to do an International Relations & History degree so therefore my A-Levels are fine.