This is a very dangerous suggest as it implies that SAML is not secure enough without pseudonymous identifiers, the use of which makes SAML deployment a lot more complicated. Pseudonymous IDs are for privacy not security. If your system requires them to be secure, you have done something wrong. Period.
I was in no way suggesting that SAML was not secure enough. However, I am of the opinion that any SSO system (including SAML) is weaker, from a security and a privacy point of view, without pseudonyms than the same system would be if it was using pseudonyms. That doesn't say or imply that it isn't secure without them, just that it would be better with them.
And I stand by my statement that had Google used good pseudonyms across relying parties, the impact of their lack of the audience restriction would have been minimal. That isn't saying that I think a system should rely on pseudonyms as their primary security model, just that the effect would have severely reduced the impact of the error.