It's not that WB is jumping on the shared universe bandwagon. It's that unlike it's competitiors, WB still owns the rights to all the DC characters. Marvel's characters are divided between 3 studios. 3 studios who are inflexible when it comes to working together. WB has the luxury of having all their character being able to show up in the MOS-verse.
It's a big bummer really. That's we'll never see Spidey team up with the X-Men (despite both operating in NY) or Hulk take it to the FF, well really the Thing in a slugfest. Sigh, Marvel comics was to hasty in selling the rights to their characters a decade or so ago.
Personally, I prefer the X-Men off in their own universe. It may be a bummer for certain storylines, but thematically it works so much better (although I am starting to get a little impatient for Fox to really expand that universe beyond just the one core team).
Overall, though, I don't see how the division of the Marvel characters really has any relevance here. Marvel Studios has created a successful shared universe despite not having access to some of their more popular characters, and most of the general audience probably doesn't even know about the split.
Also is this no jokes rumor have any serious substance to it? Like did Thomas Tull (Chairman and CEO at WB and Legendary pictures) come out with this mandate? Or is this just an exaggerated rumor? It seems like click-bait to me.
That seems a very likely possibility.