I think the point is that even people who are 100% apple are likely to need (or at least anticipate needing) to swap files with people who are not apple users.

I think being cross platform as it stands means being compatible with at minimum: Windows , Mac , iOS , android + possibly web?

I can't imagine there are many people desperate for a file sharing solution for their car or heart rate monitor, if one was developed it would probably be a niche solution possibly using dropbox as a platform.