You are correct, KVM will only be supported on the full vPro enabled systems.
I took a look at the Lenovo site and the only thing that sticks out is in the detailed tech specs:
here is the M91p for example:
you will notice about 1/2 way down the page it shows "Best in Class Manageability" and highlights Intel vPro technology
on the M81 you see that it only refers to "Stability and Manageability" with no Intel vPro reference.
I agree it could be a bit clearer, and we have created a matrix in the past to help guide users to the appropriate systems.
I will follow up with the internal team and see if we can get this matrix updated: