There already is an ARM core in the new touch bar MacBook Pros: it controls the touch ID sensor. The catch is that this ARM core does not interface with anything in user space or even much with superuser/hypervisor space.
This rumor still wouldn't permit user space applications but it would apparently work in super user space. I suspect that certain system calls from applications will be able to be executed on the ARM core, oblivious to the user space x86 application. With Apple controlling the OS, the limitations here are simply Apple's willingness to try this.
The other thing of note is that this could be a play to increase performance at the lowend. Intel's lowest power products are still dual core and adding several ARM cores to offload system tasks would be a means of freeing up CPU time for user space applications. The performance increase would entirely depend on how much background system tasks get offloaded and how much the application spends on system calls that gets ported over. At the low end this could be noticeable but on the quad core MacBook Pros, not so much.
Dual Opteron 6376, 96 GB DDR3, Asus KGPE-D16, GTX 970
Mac Pro Dual Xeon E5645, 48 GB DDR3, GTX 770
Core i7 3930K@4.2 Ghz, 32 GB DDR3, GA-X79-UP5-Wifi
Core i7 2600K@4.4 Ghz, 16 GB DDR3, GTX 970, GA-X68XP-UD4