When you refresh the thread our posts are not alternating randomly or something, so it is definitive, the forum is listing the posts in the order in which they were received.
Correct, the one listed first was processed first by the forum (and therefore has a lower message ID).
I mean, you can't rule out that the second one was actually sent first but the thread it was assigned just happened to get scheduler-suspended for long enough that a later one was handled first, or something like that. And according to general relativity, simultaneity isn't universal anyway.
