20% of people are not gay/bi.
(Under the 'Loudest Voices Represent Small Niches' part)
(They didn't cite any instances of "homosexual activists [claiming] that 10% of all humans are homosexual" and I've never heard them do such things, but whatever.)
I didn't read through the whole article, but from what I've read in other articles, 2-3% of the population is homosexual. I don't know about bisexual. I assume you got 20% by doubling the assumed 10% because of bi people. In any case, I think that there are probably even fewer bi-sexuals than there are homosexuals.