They are based on playing time projections, but to my knowledge they do not make an attempt to figure out opposing platoon splits. They may bake those into playing time projections though? Like if the Tigers were projected to face 10 straight lefties, I think their system would weight Andy Ibanez a bit more than Jace Jung, or at least it should.
What I like about projection system based systems is it does factor in who is playing. Odds change (slightly) at the trade deadline. Odds change when a star player gets hurt. They are my go to.