The article does not claim Plan9 has kubernetes built in, it claims that the namespaces of Plan 9 and the 9P protocol already had the functionality so you don't need it. It was released for production as Inferno.
It's basically a what if story. If Unix which was built around free source distribution, but only for research purposes wasn't. Linus started creating Linux because Prentice Hall kept Minix proprietary for their OS Course textbook, so he couldn't distribute code he had written on a Minix 1.5 386 system. Linus only did this because he didn't know what was going on with FreeBSD. GNU which started in 1983 was going nowhere because it got trapped in the dead end of HURD. Bell Labs were continuing research Unix with Plan 9, which instead of using the networks tools of the day and X Windows had networking and 8/12, later Rio built in to it's foundation. But they kept it proprietary and then attempted to develop it commercially as Inferno, and when it didn't Lucent sold it off. As a result Linux reach critical mass with drivers and hardware support, picked up all the GNU free software. Microsoft stopped paying licence fees for Xenix, it's Unix multitasking corporate platform and created NT with ideas stolen from DEC. By the mid 90s, tech companies abandoned paying for Unix and developed Linux as a platform for their services. And this is how we ended up where we are.
But if Prentice Hall hadn't kept Minix proprietary to sell textbooks, we would have Open Source software running on a 12,000 line Minix Microkernel OS. If FreeBSD had more that a few academics at Berkley working on it, or they got in contact with Linus when he was looking to build a posix compliant kernel from the Posix specs, Sun's Unix specs and Minix, we could have started with fully functional FreeBSD instead of early Linux.
And if Plan 9 had been released as Open Source and ran on hardware other than obscure things like than the Blit, we may have had a much smaller OS without X windows suitable for software development available. Instead it's a hobby system that's missed out on 30 years worth of developments like high level languages, games and browsers with internet, video and audio.