Source code repo is like a very extremely detailed doc. You might not be able to actually easily run it due to all of the dependencies etc, but with couple of weeks of reading, you should be able to tease back out the high level design.
I've done enough code archaeology to say that looking at the code to understand the design is a good way to understand that the two halves of the bridge didn't mate up, but there was a deadline, so...
The design from a design doc can be replicated at almost any company. The actual code is specific to the company and their exact stack.
The company's business position is similarly hard to duplicate. You can understand a company's current capital, customers and money flows. Your new company has to either outcompete for those same flows or create or capture alternative flows, and do this with different capital. Having, say, the entire source code for FedEx doesn't make it easy to launch a competitor. It's practically irrelevant compared to the network of capital investments, corporate goodwill and contracts, etc.
