If Haskell is easier to optimize than C, then it could easily be that there's some amount of programmer effort, for which expending that much effort in Haskell yields a faster program than expending that much effort in C. If that amount of effort is in the range of effort most people are able to expend on a class of projects, then Haskell is faster than C for those projects. It may even be that those are most projects.

It is, of course, not the case that Haskell is faster than C with arbitrary effort expended tuning to the specific hardware - no one is claiming that.