I don't think you understand how this works. Not only do they say explicitly that this was "an interview between Nintendo president Satoru Iwata and some of the GamePad's engineers," but also it's typical for media outlets (not just in the games industry) to report things this way. You'll also find that many "interviews" are actually done collectively rather than one-on-one. You'll see that a lot of different sites have the same responses in interviews with the same people. That's not because they're "stealing" anything. It's simply one of the common ways that media interviews are done.
Actually I do "understand how this works" as I've been getting paid to write professionally for more than 15 years now.
Professional media outlets are very strict about attribution rules and clarity. If you didn't participate in the interview, you don't use the quote as your own. If you use someone else's quote, you attribute it in the body. The main reason for not using a quote you didn't get without attribution is because you can't verify the exact words that were said.
That's why the Metal Gear Solid movie news piece on Polygon would have never passed through the copy edit department at a traditional news outlet. The author used a single source (another, competing journalist's twitter account) and did not attribute anything in the body of the text.
You are correct in saying that sometimes group interviews are done (usually at junkets and press events), but even then, the same rules apply. If you are there at the roundtable then anything that is said, you can use directly. If you are not there, then if you pull a quote from another source you attribute it when it's used.
For the IA Nintendo piece, that was generated by Nintendo's PR department. No media outlets were asking questions. Polygon didn't have someone there. That raises two red flags right away. One, it needs to be clearly attributed to Nintendo from the start. Two, there is no independent confirmation of what's said. It's a single source feature which would be held for publication by any professional news outlet.
Now granted, none of this is an issue for blogs. blogs don't have nearly the same sort of editorial quality as traditional news outlets and that's fine. Blogs are more focused on speed (as opposed to accuracy). Getting multiple viewpoints in a post is not a concern for a blog. Blogs are opinion driven by nature. And that's fine.
To be honest, none of my criticism would be raised against Polygon if they were positioning themselves as another blog. If they were, then this would be par for the course.
But they're not. The Polygon editors have said that they are the best in the business, that they have years of experience and they're going to raise the bar for gaming journalism.
My criticism comes from holding them to their words. I would love to see a site like what they describe (hell I'd love to work for a site like they describe), but so far what they've been delivering is not what they've been promising.
If they can eliminate the mis-steps and focus on achieving their stated goal, they will be a force to be reckoned with. If they keep going on the path as-is, they will simply be a Kotaku/Gawker clone.