Well, yes, but no. AMD did release this source code, but as the article tries to explain, it isn't source code that compiles into the AtomBIOS, any more than Intel's ACPI source is the source code for your laptop's ACPI tables. In both cases what we have is source code for an interpreter, because the "mysterious binary blob" isn't code for a PIC or DSP or something, but instead hardware-specific instructions for our driver running on the host.
There probably is some sort of source code for the AtomBIOS data itself, since the BIOS has to be customised for each product and there's no reason not to make that as painless as possible for the engineer assigned to do it. But since re-flashing people's Radeon cards isn't a practical solution to any conceivable Linux-specific problem there's no reason for anyone in the community to care.