If you'd want to believe their promise [2] they do plan to open up everything, but at the moment they're a closed source IM provider.
But that's irrelevant. The GP hijacked a thread about Telegram with Threema, with a three letter acronym and nothing else. For me, Telegram and Threema have the same flaw (not open). If I'd have to pick one of the two, I'd go with Telegram I guess - it's free (as in beer) and offers clients for nearly every platform I could care about.
So 'Telegram isn't open either' is correct, but misses the point: Given 'Threema FTW' my reaction was 'Why'?
1: When I wrote Why is another closed source/paid product better than this one? I tried to say (not my native language) Why is Threema, a closed source/paid product, better than this (closed source) one?