They chose their words very carefully in that article. Saying that they're "working on" a design doesn't mean they're the only ones doing it locally. It's very common to choose a design from and then work with the manufacturer to customise it - which does count as "designing".
The OEM doesn't get credited, since it's all part of the contract and their reason for existing. After all, why would Beats "break the illusion" they created?
I'm sure they worked with an OEM (they pretty much had to, to work within their manufacturing processes), but your implication that they grabbed a part off the shelf is just not accurate.
Keep in mind that doing an original design is not a terribly high bar, but yes beats were a custom designed part. They worked with an OEM the same way Apple worked with an OEM. That doesn't mean it is good, just custom.
The OEM doesn't get credited, since it's all part of the contract and their reason for existing. After all, why would Beats "break the illusion" they created?