The reason to believe that they’re very near that limit is that they’re now making cores that are almost as large as those in Skylake, and getting scalar integer performance similar to a Skylake Core M.
Apple do have the benefit of designing a mobile/tablet CPU. while Intel is using a single design to cover a much larger range of power usage and frequency. So they’re not going to be perfectly optimized for this use case. But even so, if there’s still a huge amount of slack for Apple to pick up, there should have been for Intel too. If you believe that the A11 will give a 40% single core performance boost, I think you also have to believe that Apple has some magic tech.