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).
GSA, the General Services Administration, is the purchasing, construction, and building operations unit of the US Government. They're a huge operation, with a $20 billion budget and operations all over the US. The Office of Management and Budget is a White House staff unit with about 500 employees, most in an office building next to the White House.
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.