Exactly my thoughts when I read the article - a hugely successful company not contributing to an open source project which enabled them to succeed in the first place ...
I'm a bit confused by this - I work for HAProxy Technologies and we do have an enterprise product. Many of our customers contribute code directly into the community and we backport those features into the latest enterprise stable version. This means they do not have to wait until the next shipping cycle to take advantage of a new feature. There's also a large community & knowledge base around HAProxy.
Your reasoning may be right when dealing with "closed source enterprise software" but it doesn't line up when we start talking about open source/open core.