Winamp has announced that it is opening up its source code to enable collaborative development of its legendary player for Windows

They are open sourcing, just keeping a proprietary license on it. Yes, it's weird, but it is not unheard of. The Unreal game engine's entire source code is open, anyone can read or submit changes to it. Even make changes and distribute said changes. But it's still a proprietary product owned by Epic Games, and commercial use is strictly controlled under the licensing terms. Open doesn't mean Free (as in beer), or Freedom (licensing). Those are three different things. It is just that people have associated the term open source with the entire Free and Open Source Software philosophy. But they aren't the same thing.

ZDNET is wrong, Winamp is open sourcing their code. The article is obtuse and refuses to elaborate or provide reasons about their claim that Winamp isn't open sourcing.

it cannot be open source with that level of corporate control

Why?

It not only can, we have several examples of corporate products that are open source precisely like this with this level of control.

Open source requiring a specific license is a decades old debate that continues to this day. We have like a million different licenses and people argue and bicker all the time about which ones are Truly Open source ™ and which ones aren't. It's all legalese that make most people have headaches. But there's one crux on this whole thing: Open source does not preclude commercialization of software. This is why people are proposing the term source-available software. Winamp might go for that model and the debate would still go on.