(Edit: comment below from DannyBee is authoritative.)
Google has two paths to open sourcing code: in one Google retains the copyright (but grants a permissive license like Apache), and in the other you retain the copyright but cannot work on it on Google time or Google hardware. (You can tell which category a given piece of software is under by looking at the license headers on the code; the linked software is the first category.)
It's easier to release software under the first category, even if it's just some random hack you tinker with. (I frequently hack on stuff on my corporate-provided laptop.) But if you do so then the code has the word Google all over it even if it's not something the company intends to support. I don't know for certain the reasoning but I imagine the sentence you quote helps reduce confusion about who is sponsoring the project.
"but I imagine the sentence you quote helps reduce confusion about who is sponsoring the project."
Yes. I've updated the sentence a bit for future projects. But historically, what has happened, is that people make a lot of assumptions about code Google releases and what it means for X or Y.
I've seen entire press stories about how Google has created some new product that does X or Y, when it's just some random googler's code.
(This started even before it was in the google namespace on github, and it was just a random code.google.com project).
It seems without some disclaimer, it is roughly impossible to get people to disassociate the two.
Google has two paths to open sourcing code: in one Google retains the copyright (but grants a permissive license like Apache), and in the other you retain the copyright but cannot work on it on Google time or Google hardware. (You can tell which category a given piece of software is under by looking at the license headers on the code; the linked software is the first category.)
It's easier to release software under the first category, even if it's just some random hack you tinker with. (I frequently hack on stuff on my corporate-provided laptop.) But if you do so then the code has the word Google all over it even if it's not something the company intends to support. I don't know for certain the reasoning but I imagine the sentence you quote helps reduce confusion about who is sponsoring the project.