When you install a plugin (or realy any software except maybe on IOS and android in which you have to additionaly grunt permissions) you basically give the author of the plugin an unlimited access to whatever you can do on your machine, therefor trust in the developer is something that just can’t be emphesized enough.
If you trust the developer in general there is no reason to be alarmed that he can find out whatever information that is accessable to you, like which plugins are installed.
I would not have known anything specific about the plugin even if you specified its name, but I can guess that information like which plugins you have installed is sent to their server with a license validation request or similar. It might even be written somewere in the license agreement.
But best way forward, instead of imagening things, is to just ask them.