Comparing this to Windows is silly, because Windows doesn't have anything like the X11 protocol. On Windows, running code can disable the screen saver in other ways: patching or replacing DLLs, changing system configuration, etc. No difference from a security point of view.
I'm no Windows fanboy, but this is just factually incorrect.
(1) All those operations require elevation, so unless the user has lowered UAC from the default, they will require authentication. I suppose a malicious installer could do that, but it is emphatically incorrect that any running code can effect that change.
(2) Since 7, when Windows elevates it completely suspends the old 'Desktop' and creates a brand new one for the elevation prompt. If you look closely, you'll realize that all the other 'windows' are actually just a static screenshot of what happened on the unprivileged desktop at the point where the elevation prompt was created.
So "from a security point of view", on Windows you have a specific privilege required to change the SS that is mediated through a privileged interface where it cannot be snooped/intercepted by unprivileged processes.
[ Of course, this comparison is also patently unfair -- Windows 7 was written in the 2000s, X11 was written in the 1980s. Expecting them to be comparable in terms of security is pretty ridiculous. ]