I'd say there were just as many Saints fans giving it large than Bournemouth and that post was started by one of our fans.
Whilst not rivals - it is a local derby it's so close (our closest game of the season and second closest overall in all leagues) as so many fans work with people that support each other teams.
Whether we like it or not, we're in league one competing against them. We were in that league last season, we're in it this season and the way things are going, we'll be here next season too.