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After decades of open-source software, finally we have open-source CPU.

Not yet. With a couple of exceptions, the RISC-V CPUs described in the article are all closed source.

RISC-V is an open instruction set, so anyone can implement their own design using it, often with their own proprietary extensions.

Because the instruction set is open, there are indeed hundreds of open source RISC-V designs out there, because basic CPU design is fun, easy and educational.

However, at the moment, nearly all physical RISC-V CPUs you can actually buy, dev boards etc, and devices containing them, use closed source CPU designs.