I think this misses the wider point. Google are basically saying: "we can't guarantee your anonymity so we won't pretend we can".
By enforcing a real name policy they are sending a message to those that need real anonymity: use a system designed from the ground up to give you privacy.
Anonymity and pseudonymity are two orthogonal concepts. Google could easily offer pseudonymity without making any sort of offer of anonymity. Arguing against pseudonyms with "We can't guarantee your anonymity" is nothing but a straw man.
I don't think the everyman views them as separate concepts.
People honestly believe that if you're using a pen name, then you're anonymous to those who you haven't told your real name to. Note even the terminology used here "Real Name", as in anything that isn't your government ID name is not actually a name. Pseudonyms in formal language are 'names', but not so in informal language.
"You are not required to use your Real Name to use the Service provided by Google. However Google makes no assurance that your true identity cannot or will not be disclosed even if you make use of a Pseudonym to use the Service."
... or words to that effect in the TOU would address the matter just fine for most people. For those who truly require military/intel grade anonymity, there are alternatives. For those who just want to engage with the general public without revealing their real name to all and sundry (absent hacking or subpoenas), the above should be sufficient.
If Google wish to go the extra mile and provide assurable anonymity, that's another matter.
Otherwise, this is a case of the perfect being the enemy of the good.
By enforcing a real name policy they are sending a message to those that need real anonymity: use a system designed from the ground up to give you privacy.