Something like that. RM will reduce the authorized number of F seats by a certain number and increase the authorized number of J seats by a corresponding number. So an empty F cabin might actually display as F5 and a full J cabin as J6 (just as an example). The net effect is that J seats are still for sale and they don't turn away premium business when the expect that there won't be too many F sales.
By "undersell F" I assume you meant there are F seats that are not sold because no one is buying them, instead of AA intentionally holding on to the F seats and not sell to people who want to buy them?