I believe they mean that vendors support the FOSS since it's economically advantageous for them to do so (usually bc implementing an alternative is not economically viable). The proprietary part finances their contributions to Foss, which is usually the platform that they run on top of.
There is a more detailed explanation on the economics of Foss here, for instance: https://hachyderm.io/@anthrocypher/112315622785685958, but as I understand it it's a common good that companies try to build on top of (and in some/most cases supplant with their own proprietary versions).
But yeah, I'd love to see the OP's thoughts on this.
Commenting just to get a notification in case someone answers your question - definitely interested in this angle