I highly doubt APBT for Libby, mainly due to the fact that the most common blood lines are Amstaff (which actually can be pretty slender/lean). And I've only seen 3 purebred APBTs in blue without Amstaff heritage.
Most blue pit type dogs you'll see for sale are "Razor's Edge" which started as a Amstaff bloodline (some still is) but became an American Bully bloodline.
But they can be pretty lean, some people keep them thicker since the AKC ring seems to favor it.
These are pure amstaff:
BIS, APDR Pt'd Azteca's/Rai's Thundersnow CGC

Esoteric’s Honey Badger

Esoteric’s East Coast Rounder

Retro’s If You Have To Ask

So to be 100% sure, a pedigree is the only thing that can truly differentiate the two for certain. Something that seems to hold true from my experience though is that APBT tend to have a wirey grittier look while Amstaffs look a bit "smoother" and clean cut.