An SRE does not immediately mean you have the access needed to spy on email. I'm not saying that SREs don't have the access, but it is conceivable, and likely, that within the SRE field they have higher level clearance for some employees. The response from Google in that article even points to this being the case.
That's correct. There are thousands and thousands of SREs, but gmail data is considered to be basically the most private thing in the data centers - its access is heavily gated and audited.
I understand that all SREs wouldn't have access - but if one SRE was able to gain access to look at some personal emails, surely it would be fully possible for other employees to manage it as well? I think the stance that a lot of Google employees are taking - that gmail data is secure - seems to have a lot of holes in it. The data can't possibly be that secure if this guy got fired over looking at it.
It's not possible to manage the data if no one has access. The SRE in question didn't "hack" into the data - he had access. The access was audited, so when it was abused, the SRE was fired with cause, and I think that the Google helped the prosecution.