Given that OpenCores has a ton of open-source cores on it already, why does this matter?
Open cores might get close to an implementation, but usually they break code compatibility or implement older versions of the architecture, which limits your ability to run (or update to) a modern Linux kernel or use a standard toolchain from established providers.
I think this announcement is particularly important because it gives students the chance to study a piece of RISC history. When I was in University studying for my Electrical Engineering degree, we were asked to clone an 8-bit MCU. While it was a useful exercise, everyone was dreaming of seeing a fully open MIPS CPU. We were taught it yes, in the Computer Architecture course, but never got a chance to see any RTL code.
RTL code + slides beats slides in my book so that's why I feel this announcement is so important for me, personally - but also for universities worldwide.
MIPS is a very popular architecture, having shipped in over 800 million devices last year.
Now these universities that teach MIPS in their coursework have a chance to enhance the course by giving students access to the RTL too. When those students leave university, they will have the skills required to understand and work with a commercially-available CPU architecture.
The vast majority of practical references in the book are based on the MIPS architecture (as an example of a RISC processor).