Is it literally counting number of bits? Or is it "smaller than X" in binary (same as BTC difficulty)? The BTC solution allows for finer granularity.

Same algo as BTC.

(Which is also counting the number of leading 0s in binary.)

That's not how Bitcoin difficulty works. It's not just counting leading 0 (that's just a simplification we use when explaining it to people), because then difficulty adjustment would always jump 2x...

No I mean is that an implementation of PoW for posting events?