gistfile1.sql

update events
  set reminded = all_invitees.all_reminded
from (
  select all(reminded) as all_reminded
  from invitees
  where invitees.event_id = events.id) as all_invitees

gistfile2.sql

update events
  set reminded = all_invitees.all_reminded
from (
  select event_id, bool_and(reminded) as all_reminded
  from invitees
  group by event_id) as all_invitees
where events.id = all_invitees.event_id