Inflation by definition is the increase of money supply
Indeed, but bitcoin is a complete other currency and a completely different type of money supply. It's more like gold than cash, so the bitcoin supply increasing by 13% pa is a very different (and less malicious) scenario from a central bank printing up 13% more money every year.
It's not the inflation per se that is Bitcoins advantage. It's the fact that the future inflation rate of the bitcoin money supply is
completely known in advance and can be trusted to be executed as planned.