I just tried it on a 14MB file to confirm, and Atom can open files larger than 2MB. (I also scrolled through the whole file just to make sure Atom wasn't cheating somehow, and it seemed to be working fine.)
All in all I'd say the experience editing large files still completely sucks compared to every other editor I use.
Don't get me wrong, I still love and use Atom, but they really need to come up with a solution to this problem, because it's just ridiculous. The fact that I have to occasionally leave my text editor to edit certain text files is a huge black eye that many people won't put up with, and which I will eventually get tired of putting up with if they don't do something about it soon.
One thing I think they could do is take advantage of the fact that when you are editing a file that large, you don't expect to have a particularly good experience with the scrollbar. I don't open a file with 10 million lines and grab the scrollbar to get to the spot I'm looking for; it's just not practical. Over a certain size, they really don't need to have the entire buffer in memory at once, and maybe they don't even need a usable scrollbar so much as they need good relative navigation and search. It would be perfectly reasonable in my opinion to have a "large file mode" for the editor window that has subtly different behavior that makes it not so ridiculously heavy.
Opening up a decent sized development.log did cause it to throw random errors however.