It isn’t a new interpretation. More or less this same interpretation was articulated by Justice Taft in Myers v. United States in 1926: https://en.wikipedia.org/wiki/Myers_v._United_States.

It's not, really. In Seila Law v. CFPB (2020) the Supreme Court ruled that even directors seemingly protected by for-cause language (which the FCC charter does not have) can be removed at will unless the agency in question "exercises no part of the executive power" and is "an administrative body ... that performs ... specified duties as a legislative or as a judicial aid." https://en.wikipedia.org/wiki/Seila_Law_LLC_v._Consumer_Fina...