You're talking about something different than nivia was. That comment was in reference to "write once, run anywhere" meaning that malware became cross-platform. A malicious Python app can do malicious things on any platform with Python just like a malicious Java app can do bad things anywhere you have Java.
The fact that this is exploitable through a browser plugin makes the risk of infection worse, but doesn't actually make it more cross-platform, per se.
Yes but Python also isn't generally distributed the same way. You could argue the same about the C++ redistributable across Windows and even .Net (I think 4.0 runs on XP), but malware for these are still executables. Silverlight may be the only common ground, but then that's the same as Flash, Acrobat or similar plugin.
And as you say it certainly doesn't run in the browser on so many disparate systems making watering hole attacks not nearly as damaging if there were to be Python malware in the wild (and I'm sure there are).
The fact that this is exploitable through a browser plugin makes the risk of infection worse, but doesn't actually make it more cross-platform, per se.