Jerry is talking about a distribution with three categories, two of which are nearly equal in size and dwarf the third. I agree that it makes more sense to call this discrete distribution bimodal than binary, but I don't think that's going to dispel any confusion until we see an actual histogram making the rounds.
"Ask and it shall be given ..."

What is illustrated there is 48% males, 47% females, and 5% intersex/sexless. And it's more or less based on the standard definition for sex:
As indicated, males and females have a reproductive function because they can reproduce, because they produce small or large gametes. The other category -- the one outside of those "two main" ones -- don't have any such reproductive function, and can reasonably, if unpopularly, be called sexless.
If Jerry -- et al -- want to say that the intersex are a sex then he's making sex into a spectrum of three which is basically what he is at least implying. Or he's waffling, clueless, or talking out of both sides of his mouth about what it takes to qualify as male or female.
And, as suggested, that "Reproductive Function" graph -- including the sexed and the sexless -- might, at a stretch, be called bimodal. But there really aren't any intervening "sexes" between "male" and "female" so apparently not consistent with mathematics terminology about peaks, about values on either side that are of lower frequency.
ETA: Jerry has apparently crossed the Rubicon here by explicitly asserting that the intersex are neither male nor female, i.e., they're sexless:
In fact, you don’t seem to understand the issue under discussion. Those 1/6000 individuals are intersexes, neither male nor female. They are almost never fertile (two hermaphrodites in history are known to have produced either eggs or sperm but never both) and they are NOT a third sex.
If ever there were a headline designed to get my dander up, it’s this one from a San Francisco Chronicle op-ed (click screenshot below to read, or see it archived here). The author’s se…
whyevolutionistrue.com
Though he still seems somewhat unclear on the concept -- many of the intersex are sexless BECAUSE they don't have functioning gonads of either type -- ergo, infertile.