Uh, no. You may want to read the article again. It wasn't the author's conjecture, they stated as fact that they were looking for British actors.
There are probably a lot of thoughts about who should be cast in these roles and how they should be portrayed, but we've learned that Marvel and Joss definitely have a few ideas already and one of them is that the duo are going to be younger than the rest of the team.
Although Joss is still writing and probably not ready to send anything out to actors, for the two roles, they're generally looking for actors in their 20s and we've heard word that the "prototype" for their Scarlet Witch is none other than Irish actress Saoirse Ronan. Except something to note is that they're going "British" for the role rather than trying to have actors playing Eastern Europeans.
As it happens, Ronan just turned 19 last month and she will be turning 20 sometime during the time they would be filming the movie next year though she's also already been playing more mature roles. If she is Marvel's "prototype" for Wanda, why wouldn't Marvel just outright make an offer to Ronan once they have a script done? Ronan has those mesmerizing eyes that would be perfect for the Scarlet Witch, too.
As far as Wanda's brother Pietro--now we're assuming they're going to use the same first names from the comics even though they're played as British---he's only described as being "euro" and "edgy," but there are plenty of British actors in their 20s who could fit that bill. Come on SHH readers, what actor could hold their own against Ms. Ronan? Please don't mention either of the actors from The Host, though.
Considering that they did end up in talks with a young British actor for Quicksilver, I think it's safe to say that the article wasn't BS and the source was legit. As for Ronan shooting it down, she did that before Joss had finished the script and they weren't really in talks with any actors. Even with Aaron there weren't reports of him being in talks/Marvel's first choice until after Joss said he had finished the script back in June or early July. There haven't been any interviews with her since then so it's impossible to know where it's standing at now. And like I said it seems like they're casting Quicksilver first and then movie on to Scarlet Witch considering we haven't heard anything about her.