Open source is meant to be a synonym for free software that doesn't explicitly mention the "freedom" part of it. Open source is meant to be a new name for free software that doesn't scare businesses away. The vast majority of software vetted as open source by OSI is also considered free software by the FSF.
Personally, the emphasis on freedom matters to me. If some business is scared away by freedom, I'll pass up on that business and take the freedom instead.
Free Software is not synonymous with GPL. GPL is a specific kind of free software with copyleft provisions which guarantee that users (can) get the source. The post you're responding to seems to argue not just for the role of free software, but for that of GPL specifically.
Personally, the emphasis on freedom matters to me. If some business is scared away by freedom, I'll pass up on that business and take the freedom instead.