Let's say I have an open-source project licensed under Apache 2. The grant allows me to include the extension in my project. But it doesn't allow me to relicense it under Apache 2 or any other compatible license. So if I include it, my project can't be Apache 2-licensed anymore.

Apache 2 is just an example here - the same would apply for practically any open source license.

The one place I imagine it could still work is if the open-source project, say a sqlite browser, includes it as an optional plugin. So the project itself stays open-source, but the grant allows using the proprietary plugin with it.