That's not quite the case.
You can easily sharpen a properly tempered draw knife with a file. The file shouldn't really dig into the steel, but it in no way should skip across it. That would indicate the draw knife would be in the mid 60's on the Rockwell scale, when nearly every one I have had in the shop has been in the mid to low 50's - ie softer than the typical hunting knife.
I do have one newer knife that's in the high 50's, but I still did the initial re-profiling with a file.