"the grappling with the concrete that architects really do. They make models, they draw diagrams, they find out late in the game that they've routed two pipes through the same space (and they let the contractor sort it out). "
most of the houses that Marx lived in in London had no architects. They were built on the system - common in Victorian England - from pattern books. So-called 'speculative builders' would put together a team, take time building the first house on the street, from a pattern book, and having taken their labourers through all the problems that might turn up, they would just carry on building houses down the street in a terrace until they ran out of money. When they sold them, they would start up again, often with a new team.
I don't doubt that Marx knew that plans diverged from execution. Certainly that was his own experience as the many versions of the plan of his magnum opus demonstrate. His point, though, was that human beings have the ability to consider their actions (make a plan) and reflect on them before doing them. The bee, we know, has no such inner life (or we would have heard about it) but acts according to a set of genetically laid down commands (what Marx would have called natural instincts).