I'm in my 30s and I obviously had friends when I was late teens, early 20s who I was immensely close with, did everything together and have so many memories with. Some of them, I haven't really spoken to in years. The odd message here, facebook comment there. God forbid if anything happened to them, I'd be devastated, and sad for their children and families, but would my day-to-day life be impacted by the loss? Probably not. I feel like that's how it is for the 1D boys. Niall and Harry's posts talking about his energy and passion, the time spent together in the band, public condolences to his family... It's indicative of "we made some great memories in the past but we were no longer close" which is completely understandable. In my eyes, they had no choice but to say something and they have. Zayn and Louis' are far more raw and personal.
I would wonder whether they have all actually been in contact with eachother. I would like to think so but the 1D post could have easily been put together for them and they've each okayed it separately.