[gsg]

Those compilers use CPS as an intermediate language - a bit like SSA form for functional languages - which is (mostly) independent of supporting first class continuations.

Support for first class continuations is a matter of tradeoffs. It's quite reasonable for an implementation to choose 'slow' continuations (in which call/cc copies the stack) in order to be able to use the more efficient native stack for function calls.

(EDIT: and in response to the linked post, it is quite possible to efficiently compile what would be contifiable functions in direct style - see the recent paper Compiling Without Continuations in which the GHC hackers do exactly that.)