I don't know how I could prove that one way or the other. The overall trend seems to be towards relaxing the need for warrants, although there are important exceptions.
If law enforcement have their own backdoor, that seems to suggest less judicial oversight. I personally trust Schneier to depict the state of affairs accurately, probably using public and non-public information.
We all know services like Gmail can be subpoenaed by court to provide information. Google is big enough for this to happen to them on a daily basis, so it's very likely they have a an automated procedure. For all we know, Schneier may be talking about this.
If law enforcement have their own backdoor, that seems to suggest less judicial oversight. I personally trust Schneier to depict the state of affairs accurately, probably using public and non-public information.