Aw man, I'm caught up.
My interpretation is that they literally are that close in terms of what they are to each other. Some may say that it goes 'beyond friendship' (as in, romance) but I don't really think of it that way. Due to how odd they are as people, they similarly have a very odd relationship that cannot really be described as 'dating' or 'just friends', they are simply 'them'. This is a platonic relationship taken to the extreme whereby most would see it as 'romance', when there are likely none of those kinds of feelings involved.
It's very refreshing to see such a straight-forward thing like this, all the ecchi aside.
My favourite part of all this is definitely how nonchalant they are. They aren't bothered by each others' shenanigans, in fact they contribute equally to such situations. Others see them differently ("JUST GET MARRIED ALREADY") but that doesn't stop them. There's already a lot of 'orthodox' Romance manga, so this is great. I hope it doesn't change. I hope by the end of this they stay the same. This could be a great Comedy manga.