Hm, I would read this more as “Just to let you know we’re having a party on the roof terrace this weekend. So if you were planning on having one too, or wanted a quiet afternoon on the roof terrace be aware that we will be there, doing this!”.
I wouldn’t think it meant you couldn’t use it, or they’re apologising for anything… just giving you the courtesy of letting you know this is what they’ve planned in this shared communal space. I’m thinking i’d write a similar note and then be faced with the question “Do I invite my neighbours too? Ugh, don’t really want to… and they probably don’t want to come anyway… but if i ask them they might feel like they have to come? … maybe i just won’t mention it and that way they can do what they like, yeah, good idea”