Perfectly sensible policy and I for one appreciate insiders clarifying misinformation especially on such issues, while still notifiying their potential conflicts.
Disclaimer: I am not a googler, opinions are what google tells me to think.
Some comments I see (not speaking of this thread per se, but in general on HN) are really close to conveying internal company information. Which is beyond mere opinion and only available to someone inside the company. Some people clearly use the disclaimer to elevate their "opinion" to the level of authority.
It's not about access to information. It's about removing the possibility of being confused for a spokesperson of Google's.
This is in contrast to comments you see here like "hi, Ben Foobar from [company being discussed] here", where the company is being formally represented and communicating with HN.
Naturally Google doesn't engage this way on HN, but the disclaimer is written out of habit and an abundance of caution.
Source: I work at Google
Disclaimer: Googler, opinions are my own.