Before I begin, I am a fan of Halo Wars and would like a Halo Wars 2.
That being said, why would 343i buy the rights to it from their father company (Microsoft) who has always held the rights to Halo Wars? Firstly, Microsoft would never sell the rights to anything Halo related. Secondly, 343i wouldn't NEED to buy the rights from Microsoft because they are owned by Microsoft. 343i, unlike Bungie, is not an "independent contractor". 343i was created by Microsoft to help handle Halo. When Bungie decided that they no were going to separate from Microsoft, it was decided that 343i would be the division that would take over development of Halo games in Bungie's stead.
The only thing that has prevented there being a Halo Wars 2 thus far is finding a new game engine. There are copyright claims on the previous game engine, and no one really knows what to pay who because of Ensemble's disassembly. The only solutions to this debacle are a lengthy and possibly costly legal battle over rights to the engine, a development of a new engine, merging the former employees of Ensemble with 343i (which probably won't happen. Most of them are happy where they are now), or just putting Halo Wars 2 on the shelf for now. Microsoft has obviously opted for the last of those choices, because while Halo Wars was the best selling RTS on a console so far, it didn't sell nearly as many copies as other Halo titles that were FPS. So, it makes sense to focus on the FPS parts of the Halo series than the RTS parts of it.
As far as development of it beginning this summer... That's highly unlikely. Most of 343i's staff is working on Halo 4 and the rest are either writing stuff for Halo 5, working on Halo Reach's multiplayer, or enjoying their vacation because they've completed their tasks for Halo 4 (the soundtrack and parts of the recording teams). If they were to begin development on Halo Wars 2, they would be undermanned and both Halo 4 and Halo Wars 2 would suffer from it.
As far as proof goes, I'm not sure where you read what you read, but the fact that you didn't cite a link to the place that you read it seems fishy to me. For future endeavors, you should remember to cite your source (by posting a link to the original information) and give a brief summation of what's written at the source before you write what you think it means.
Also, I would REALLY like to see where you read that 343i purchased the rights to Halo Wars. Especially since their parent company is the one who owns the rights.