Ann: Quarterly Activities Report - December 2020, page-94

Bolt-on just means that it can be added at some point after the completion of the already-planned plant, without requiring the plans for the aforementioned plant to be significantly amended, and that the bolt-on is not required for the plant to serve its function.

Think of it like the nozzle on the end of a hose - the nozzle does something, but the hose works fine without it, and its pretty straightforward to whack it on there no matter how long you wait between the installation of the hose and the nozzle.