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
The difference is that GPL is claiming to not be proprietary...