Before you read this, keep in mind I’m not a lawyer.
Even if you make a paid game, those assets will still be included, and people would be able to get them, even if they haven’t paid for them.
I think the important part here is the license. It’s fine if people get hold of those assets, but you have to make sure they are aware that those assets are not licensed for commercial use. That means they can’t publish any work that includes those assets. Unfortunately this can get difficult to track, I’m not sure if there is an easy way to deal with it.
Keep in mind that it doesn’t matter if your project is open-source or not. I can access assets of a closed-source AAA game I’ve bought, that doesn’t mean I can use them on my projects.
If in doubt, check with the original creator, to confirm what their intention was when they were selling the assets.