How is that even somewhat ambiguous? Personal and, perhaps, skirting the outer edge of what a remarkably parochial person might regard as quirky, but he laid out a drink preference specific enough someone has a shopping list and doesn't have to guess. Which is what a rider is for.
You don't even need to buy him tea; he explicitly says he always brings his own with him.
So if I provide tea he likes, I don't have to provide milk and sugar. If I provide tea he doesn't like, I have to provide milk and sugar. Unless of course he chooses to drink his own tea, in which case my tea, my milk, and my sugar will all go to waste.
This is fine, as long as I know precisely what kind of tea he likes. But that's not specified, so I just gotta guess what he might like. And then buy some milk and sugar, just in case he likes my fancy tea just enough to drink it, but not enough to skip the milk and sugar.
Really I think he's just writing his opinions on tea off the top of his head.
Having had experience organizing lots of public speaking/platform events, I can assure you that "please provide milk and sugar" is one of the easiest-to-comply with requests I've ever seen. He doesn't specify that it must be soy milk, or 2%, or half and half. He doesn't say it has to be natural cane sugar grown on the sunny side of a hill and harvested by certified sugar cane naturalists during the summer solstice. He just wants milk and sugar; beyond that he leaves the details to you.
Worst case scenario is that he doesn't use them, in which case you're out what, two dollars? That is peanuts compared to what some riders cost you by specifying elaborate A/V and lighting setups, specific vendors and contractors (i.e. friends of the speaker) you have to deal with, luxury transportation and lodging, et al.
As to the "what kind of tea" issue, he helpfully solves the problem for you -- he'll bring his own, which he is guaranteed to like. So there's no scenario where your failure to choose the right tea will result in him being tealess. Strictly speaking the only thing you're on the hook to provide is hot water, which is free and easy to scrounge up at short notice.
It's really a remarkably stress-free document, as riders go.
He's basically saying, "if you provide tea provide a full tea service or I likely won't like it." If you don't want to provide that, don't.
If that's too complicated he repeatedly says "email me and ask". "Hey RMS, what tea would you like us to buy" would do it. As would giving him $0.75 for the tea bags he brought, if you feel the need.
You don't even need to buy him tea; he explicitly says he always brings his own with him.