The wowhead post is a time capsule and incorrect. The original blizzard post was updated to include that the m+ cut off time was 10 PM PDT as well.
“The current season of Mythic+ content will end at 10:00 p.m. the night before maintenance begins in each region. This is 10:00 p.m. PDT for North American realms and 22:00 CEST for European realms. With the season’s end, ratings that qualify for the 0.1% “Shrouded Hero: Shadowlands Season 4” achievement will be recorded.”