to reply to myself, if you have a second interface configured for failover, it actually tests against ping.ubnt.com. I bet every single time my ATT fiber has "gone out" for a minute or two at a time, it's been bogus.

  root@USG-PRO-4:~# show load-balance watchdog
  Group wan_failover
  eth2
  status: Running
  pings: 2
  fails: 0
  run fails: 0/3
  route drops: 0
  ping gateway: ping.ubnt.com - REACHABLE

  eth3
  status: Waiting on recovery (0/3)
  failover-only mode
  pings: 1
  fails: 1
  run fails: 3/3
  route drops: 1
  ping gateway: ping.ubnt.com - DOWN
  last route drop   : Fri Jul 17 17:32:58 2020