It does make a reference to it further down from the synopsis. The difference here I think is that most of the PHP community seems to abandon support for Hack,

It does make a reference to it further down from the synopsis. The difference here I think is that most of the PHP community seems to abandon support for Hack, so Hack (as far as I can see. I am happy to be corrected) in the wild is more or less contained to Facebook and its properties. I think a couple of large companies like Slack use it in some places too, but its mainly been disregarded by the broader community (none of the largest frameworks support Hack anymore, for instance). Which makes sense why Hack is no longer developing with PHP compatibility in mind.