The way I see it (no comment on right or wrong), Preferred Plus is just the marketing term for a paid bundle by of; seat with extra leg room, drink, and pre-boarding.

MVPGs get open access to all seating, their free drink because they are MVPG, plus their established pre boarding group.

MVPs/AA EXP and PLT get open access to seating, their established pre boarding group. The drink just continues to be unbundled.