Them being in the same generation doesn't mean a thing. Do Black/White and Black 2/White 2 happen at the exact same time because they are in the same generation? No. By your logic, the latter should be in a separate generation because of time difference.
Before you go on a rampage stating something as WRONG!, please bring up a real argument yourself instead of just saying something along the line of 'this site says so so it's right'. Bulbapedia, while it has an abundance of knowledge, also lacks much solid proof to indicate a specific placement of RSE in the timeline. That part is just pure speculation at this point.
As I've mentioned before, what do you make of Plasma not making any mention of Magma and Aqua's actions in BW(despite what they were trying to do was a lot more notable than Team Rocket), while former Aqua/Magma members do appear in B2W2 to mention it? That shows that RSE could have taken place between BW and B2W2 as well. It can also be argued that Steven referring to himself as champion in HGSS could mean that neither us nor Wallace has beaten him for the spot yet.
Before it's brought up again, direct trading is also little proof because, once again, B2W2 breaks that mold by allowing direct trade and battle with their prequels, despite happening two years later. Trading is also more of a game mechanic than a plot point anyways.
On top of that, Gamefreak has already made a mention that the games come out as they come out, with no real pattern in mind while they make them, so Hoenn and Kanto(brought back) being in the same generation of games could be simply coincidental and/or just a retcon to make up for the fact that RSE couldn't connect with RBY/GSC.