I estimate an 80% chance that the base rate for polyamorists, if known, would directly give rise to a probability of 50% or higher for the basic claim being true for three or more persons fairly closely associated with MIRI.

The vindictiveness on the linked page strongly suggests an insider; it is hard to imagine someone hating them that much, without at least having met them personally. Likewise, some of the expressions (such as "with the possible exception of one") are more suggestive of someone's actual belief, rather than a clever lie.

Overall estimating a 65% chance the webpage is basically correct in terms of the substance of the claims, although certainly false in terms of the black and white painting of the situation.