Isp is an efficiency, but it is specifically efficiency with respect to fuel usage. As this has commonly been the main measure of non-electric propulsion it is

Isp is an efficiency, but it is specifically efficiency with respect to fuel usage. As this has commonly been the main measure of non-electric propulsion it is still valid to invite the comparison as part of the goal is still reducing mass by reducing fuel usage. It does seem the efficiency they are referring to is efficiency in ionization. And I agree it is a bit odd that they bring up the claim of "superior efficiency" and quote the power usage of the thruster (50W) without any notes about the actual thrust capability that it provides.