I've started open sourcing more things lately, mostly in the realm of things that solve a problem I have that might solve someone else's problem. Sometimes I build tech for tech's sake that improves my software or apps, but it isn't a product in itself. Frameworks, libraries, templates, and tools all tend to fall into this category I think and github is a great example is open sourcing these things.
You shouldn't open source your product itself unless you want to be a service company. I personally don't care to become a service company, so I keep the "product" and the data behind it closed, but the frameworks, tools, etc. can and should when possible be open source.
The product and data in this case is your "core" that the article is advocating to keep close to your chest while outsourcing the non-essential components that would attract interested parties to your work.
You shouldn't open source your product itself unless you want to be a service company. I personally don't care to become a service company, so I keep the "product" and the data behind it closed, but the frameworks, tools, etc. can and should when possible be open source.