If you wrote 9,990 lines of proprietary code and included 10 lines of proprietary code from another company, I imagine that your entire program would have to be approved of by the other company in writing, or you wouldn't be able to distribute it.
If you're a business, the license effectively says "you can't use this software" though, because as discussed previously it makes all the software that it touches into GPL software so you can't sell it, which is the point of a business.
What is there to understand? We sold GPL licensed extensions to a GPL application, and made money. All perfectly valid under the license: https://www.gnu.org/philosophy/selling.en.html
What's supposed to be the difference here?