If by "business model" you mean, "how do we keep the lights going", that's fair. However, that's not the primary mission of Let's Encrypt.
That was roughly my read. While "business" is frequently used as a synonym for "for profit company", that's not the only definition. Someone saying "that's none of your business" is unlikely to be talking about S corps.
> However, that's not the primary mission of Let's Encrypt.
It's the means by which they accomplish their primary mission. Can't encrypt the world if you can't keep your servers on.