What a redundant theme/ round. The badge and jersey already represents the community - or so you'd think...
This is just an excuse for another jersey, and a rip-off of their late 90's one at that.
Manly have a long term deal with URM (who have been a partner of the club since 1997) which allows them to have one home game a year where they are in the primary sponsor position. As any jersey change, whether that is change of sponsor, design or what not, counts towards the total number of jersey variations the club is entitled to, the two parties have decided to use it as an opportunity to mix up the design.
This isn't a new thing, as
@Timmah pointed out earlier, the Community Day concept has been going for quite a while now, with I think 2016 being the first (photo below).