I guess that means that if the MC is in an open relationship with both Jaye and Mallory, there will be an added option to be with one of them, but probably not both.
Frankly, I can accept almost anything they do, as long as they leave the poly route through Jaye available.
It is also my guess that there will be no MFF scene with the MC, Jaye and Mallory in the Epilogue, although that is something I would certainly LOVE to see.
By the way, SFS has confirmed in a reply to a comment on their Writing a wrong post that 'casual route' is the same as 'open relationship route'.