So then why offer the source code for modification?
Setting a license is not the same as offering source code- especially something like GPLv3. That particular license, for better or worse, is a virus.You can't choose to set that on someone else's source.
It says something specific about what you're trying to do, and you can't decide that for someone else.
It's not originally your code. It's not originally your project. You can contact the developer to clarify their position. But not being able to find that developer, and/or not getting a response does not leave it open for you to decide that. See Circle Dock for the example of what bad things
can happen when you try to do so. This is a big deal for a lot of people. And when things are a big deal, making unilateral decisions for others isn't a good thing to do, no matter what you might think. Their reactions might surprise you.