Hi, that is correct, but my point is that the z-index value is arbitrary and each website might set a different value. You can set your own with some custom CSS if you need to. A plugin needs to work with the majority of websites but there will always be cases where the CSS or settings need to be adjusted manually. So I don't think the plugin needs to be changed in this case- for example, what if some theme sets a z-index of 100001? It'd end up being a crazy race to the highest z-index value. You can always manually adjust the z-index to suit.
Hope that clarifies.
Regards, Richard