Someone else feel free to correct me on this if I'm wrong, but yeah, you're seeing some people reply with "Hiatus!" as a courtesy since their hiatus was over 2 weeks within December (Example: 13th December - 30th December) while this player's hiatus was 12/21 - 1/20, which is only 10 days in December and thus doesn't exempt from providing their December activity.

From the AC post:

"Those who have been on hiatus for more than two weeks during the AC period will be exempt (though we would like you to check in, if possible!)"

So only people with a 2 week and above hiatus within 1st to 31st December have no need to reply at all, should they be lacking the means to do so.