It's not fully in house. The idea is to centralize management of services (both in-house and external) for budgeting reasons, since MAX is part of GSA which is part of the OMB (Office of Management and Budget).
Btw, GSA and OMB aren't connected the way you describe. They just happen to be the respective parent agencies of 18F and USDS-HQ. Here's a tree.
POTUS
├── GSA
│ └── 18F
└── OMB
├── USDS-HQ
└── MAXGSA has forklifts. OMB has clout.