As noted, this is the correct answer. The confusion seems to stem partly from the fact that Basic Economy fares all use fare basis codes that start with coach class fare codes. However, the coach class in the fare basis code is used solely for dual inventory checking -- there must inventory in both B class and the coach class listed in the fare basis code in order to book the fare. However, your benefits and earnings are dictated entirely by the booking class which is Basic Economy B. Domestic FC fares uses same type of coach fare basis codes and dual inventory checking. AA uses the second to last letter of the fare basis code to indicate actual booking class for such fares.

Delta uses dual inventory system for it's Comfort+ fares, in addition to Basic Economy and domestic FC. However, DL uses second to last character of fare basis code to indicate class of service, rather than booking class (so you can't always tell the booking class from just looking at the fare basis code with DL fares -- although Basic Economy class of service will always book into E class on DL)