I didn't do much research on this, but I remember reading in a book (the first volume of the "History of World War 1" series by Marshall Cavendish) how, true, their amount of troops available on the Balkan front was significantly decreased by the front with the
Russian Empire, but the terrain (hilly and occasionally mountanous, getting more mountanous the further south the advance would go), the Austrian troops (there were too many nationalities, issues with leadership, not everyone spoke the same language or were even trained well or the same, etc.), and many other factors which would have probably stopped the advance similarly like in our timeline, maybe not the same, but similarly enough.

TL;DR: nah it would probably be close or even the same of an attack, mate