I incorrectly used the term super-heated steam. Super-heated steam is actually dry steam and occurs when all the water has been removed from the system, and we're really talking about wet steam here.
Wet steam is by definition hotter than 100 C at sea-level, otherwise it would not be steam. Yes, the combined average temperature of the liquid/medium and the steam would tend towards 100 C because the steam would start giving away its heat energy to other surfaces while the liquid would continue to boil, but the steam itself would have to be above 100 C. The medium is also not just water - it contains sugars so it would increase the boiling point a little (though probably not by much).
Yes, a pressure cooker would allow both the vapour and the liquid to get hotter. As you say, the microwave is not sealed so you can't maintain the temperatures, but the microwave method still works. This is where I took my instruction from and it worked for me:
Maybe you can't technically sterilise the inside of the microwave, but no living organism would be able to survive for very long in a microwave anyway.
Unless you have access to a clean room, everything you do to sterilise is just a best effort, and if it works, I say just go with it!
Edit: I stand corrected about the temperature of steam - it's not necessarily hotter than 100 C. However, a gram of steam contains more heat than a gram of boiling water, which is why steam burns can be so severe. I also learned that microwaves often super-heat water, which I was not aware of.
Mxptrsn 
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