To the point: two issues are raised.
First, there needs to be a fourth (ground) wire connecting the main panel with the sub-panel.
Second, the code conflicts here with common useage. The "main disconnect" breaker in a panel is not the same as what the code calls a "main disconnect." The NEC 'main disconnect" refers to the point where the utility lines ("main service") arrive at the meters. There can be no more than six meters without there being a single handle to disconnect all of them.
Everything after the meter is either a "feeder" to a sub-panel, or a "branch circuit." A sub-panel must have a disconnecting means, but this need not be at the sub-panel (it can be at the source panel). Under certain circumstances (called "tap rules") the overload protection need not be at the source.
As you can see, a journeyman is paid for his training, experience, and judgement- as well as his time. You've got to respect him for that.