I don’t know anything about HHVM, but plausibly the goal of it is “help Facebook” rather than “be a faster PHP,” and people who came for a faster PHP and are not Facebook are let down by it.
Non-optional strong types is one example where it benefits Facebook, but likely isn't great for those hoping for a "faster PHP". The blog post makes it sound like HHVM will only support hack, and not PHP, sometime in the near future.