Because character statements are canon, simply put, we know as a matter of fact that the Sith war machine had been halted in it's tracks once the Republic got it's act together, they even pushed the Empire right back to Korriban, the Seat of the Empire, it took Malgus and others pulling a pincer movement to force the Republic back a few systems, the Empire got blitzed before the Republic stopped, then eventually the Empire(Revan) made the Sacking of Coruscant and the Treaty happen.
The Republic definitely were holding their ground in the mid-rim, the only reason the Empire got all those extra systems is because they'd taken the Republic's capital hostage and 'forced the ink on the paper', to coin the phrase.
After that however, the Empire retook Ziost and began pushing the Republic back again. Taking key worlds such as Serenno. I believe the Imperial advance would have continued if not for the TOC.