I based my 100,000 on the figure quoted by a 2004 article I linked earlier suggesting 300,000, note I dropped the number significantly to well under what it likely. The 2.5% of beer consumed as homebrew does not imply 2.5% of Aussies homebrew, it is a (very) weak implication on the number of homebrewers. However it serves to give an approximate order of magnitude, and matches up approximately against the 2004 article. Again, note that I have heavily discounted these figures in a manner that goes against my argument.
I'm just using what information is readily available to get ballpark figures and I think I've done so in a logical manner.