It's an out-of-band locking mechanism. In particular, one which mentions "you should consider fencing tokens" merely in passing instead of baking them in from the start.

The only reasonable correctness-oriented view of that is "lol". It's not worth throwing a Jepsen-like test at it, the fundamentals aren't even slightly sound, merely "usually good enough". Whether that's worth it for [use X] depends on that use - often yes!