Sounds to me that is happening exactly the same as Delta with Endeavor.
Why would United award Ejets to a company that doesn’t operate them when having others that already do it efficiently?? Nobody would put the time and effort of a learning curve if you are not thinking about making a long term investment with them. XJT being now theirs (though legally it isn’t) they can control who they hire, how they train them and how they provide the service to the customer, which is the reason why I think Legacies are moving away from the “outsorced flying” model.
I think once XJT has the 175 program up and running they will start bringing outsourced flying of 175’s in house (with XJT) just like Delta did with Endeavor.