Compared to more traditional messaging protocols it could seem that way depending on various factors like time zone differences and how often devices can be online.
It seems like in general, 1:1 conversations will require both participants to be online simultaneously to communicate. Group conversations can have any online participant act as a relay for new messages to offline participants, more or less.
Check out their documentation, particularly the article on how the distributed network works. Also the FAQ is massive! I wish I had the time to read about this in more detail right now
Interstellar_1