Just recently swapped a 170k mile 5.3 into my truck (got a good deal on it and my last 5.3 blew up). Driving down the road today, it revved way up and I thought the trans was slipping, pulled over and shut it off, checked fluid and everything was fine so I tried to start it and it sounded like the starter wasn't engaging. Put my phone under it and recorded a video and this is what I got.

https://youtu.be/L2wpUZK1Q8g

Unless all the flywheel bolts fell out somehow which I highly doubt happened as I used thread locker on them, would this be a snapped crank?

 

Not likely a broken crankshaft, The reluctor wheel is at the rear of the crank, If the crank broke the reluctor wheel would stop spinning and the engine wouldn't continue to run, Flywheel bolts missing or sheared off.

 

Did you use the factory bolts again or put it together with a impact? Your gonna be digging broken bolts from the back of the crank. Once you get it all removed install some arp flexplate bolts. Use the recommended sealant and use a torque wrench to tighten it down.

 

The flexplate may have cracked around the crank flange. I've seen it happen before.

Breaking a crankshaft is a pretty rare occurrence, especially on a stock engine.

EDIT: After watching your video, I'm almost positive this is what happened.

 

^^^ This ^^^

When that happens, the starter spins the torque converter and the remains of the flex plate, but none of that is connected to the engine anymore. It's pretty common actually.

That'd be my diagnosis as well.