XF has an automatic system. If the author doesn't login within 3 months I believe, his resources get deleted automatically. When he comes back, they will be undone and listed again.
So, it is just a precaution so people don't buy unsupported addons.
In your case you don't need to uninstall the addon if it works. It might work from now on for years to come or break when XF updates its software. If you are still unsure, just disable the addon and enable it when the author is back but I run addons which are not sold anymore for years now. They still work (fingers crossed). It depends if the addon breaks or not when XF updates its software (and you update your installation).