I just told you why they use the word community though? A community is a group of people in the same place. Subreddits have been referred to by the people in them as a community for years. People on Discord talk about their “community” on their servers. People on Xbox Live are a “community”.
It literally has nothing to do with Lemmy. Reddit aren’t threatened by Lemmy. Look how the protests went - they all flickered out and died with a whimper. Reddit is business as usual. Pretty much every sub is back to normal as if nothing ever happened because nothing basically did happen.