Doing engineering prototyping and bringing a polished product to market are very different things.
Tesla doesn't have surplus employees, any minute spent to bring the AWD Model S to market must cost time from work on the Model X.
It may not be engineering, it might be logistics, supply chain planning, process, building the assembly line, integration testing, crash testing.
It might even be true that the vast majority of the work is done - and that trading 1 month of delay on the Model X means bringing AWD Model S cars to market a year earlier than they otherwise would.
That would still likely be a bad business decision. One reason is that everyone knows about the Model X and that it is late - but there are no expectations for an AWD Model S.