I watched the video replay and have seen bigger celebrations from players who tackle the QB or interrupt a pass play.
The NFL admitted that the call was wrong, that the activity should not have warranted a penalty.
In this situation, the ref is the one who made the error.
I’m not watching any football this year, but my local talking head said Reid immediately told him he couldn’t slide through the endzone like he did. I can’t speak to it since I haven’t seen it, but my brother did say he slid like 15 feet before kneeling down.