I have a little bit by way of Haskell chops, and I'll venture that the performance of the Hack typechecker is a very big deal, and it is in a different breed than the turnaround time you get from snap or yesod (or Java).
Edit: I misread what you wrote. I thought you were saying there was something fundamentally different about the type checker, but more that reload experience is different then what you get with Yesod.
"Type checking is faster than other statically typed languages" is quite different from "other statically typed languages don't offer this workflow", which is what was claimed.
Edit: I misread what you wrote. I thought you were saying there was something fundamentally different about the type checker, but more that reload experience is different then what you get with Yesod.