No. It doesn't prove that they're working on it. It shows that they're thinking about it and are collecting information from us. I do agree that the info gleaned from that thread is unlikely to help them with an expansion though. But since we're only on version 5.1 it would make much more sense for them to be working on improving DW with a 5.2 patch before an expansion pack.
As someone mentioned, pdox has two development teams, both of them have finished their projects recently. Conceptual phase for new game in many development studios begins even before current game is released. I believe that is true for pdox as usually they announce their new games few months after their previous project is released. And always when new game is announced the conceptual phase for this game is over: the developers know which new ideas are in the game and which are out because they have been working on it for some time.
So they are probably working on one big project - not because of any competition, but because its most profitable for the company. They are probably also working on one smaller project (maybe an expansion or minor grand strategy game like Sengoku or Rome - for second development team) and few little projects (patches for CK2 and EU3, maybe some DLC).
I believe that the biggest project for main team would be EU IV or some new franchise game. It is most logical, since HOI3 and Vici 2 are still brand new games.
So, yes - we can guess that they are working on EU IV. It usually takes pdox from 18 to 24 months to complete biggest and most important projects – I guess it would be 24 months for EU IV. So game would be released in 2014.