I think it's actually a pretty unsurprising assumption. Slack is significantly more vulnerable service than a lot of other chat applications due to the fact that it has a native webclient that allows cookie based authentication. Every chat program in existence is going to use something extremely similar to maintain user sessions - having either authentication credentials or (preferably) an access token written to disk to allow users to avoid re-entering passwords every time the program falls out of memory.

I think with slack the danger is exacerbated by the fact that this value gets stored in a cookie that is a lot easier to gain access to - and, once you have it it has a much better documented format than whatever MSN Messanger might roll in-house. We're essentially just talking about an added layer of security by obscurity when it comes to the format - but actually nabbing a cookie is easier than gaining arbitrary file access.