Its more like "immunity for any action within the realm of presidential power, regardless of motive or criminality". Accepting a bribe in exchange for a pardon would be A-ok on the basis that pardoning is a “conclusive and preclusive” authority of the president.

> The opposite holding, where they are liable for everything, would be untenable.

Literally no one was arguing for this and there are much more reasonable interpretations of presidential immunity you could compare this one to.