As with many definition terms there are multiple answers to what is a library and what is a framework, and the phrasing you've described is one way, but another axis that is probably closer to why Bootstrap chose the name "Framework" over library is: "do one and only one feature" versus "generously include the kitchen sink".
Often in CSS terms a CSS library is focused on a specific "vertical": a grid library provides everything you need for a grid. Whereas a CSS framework generally focuses on providing a little bit of everything ("horizontal"): Bootstrap has a grid system (library), but also a button library, a modal popup library, etc, all bundled together in a one-stop shop for developer convenience.
Often in CSS terms a CSS library is focused on a specific "vertical": a grid library provides everything you need for a grid. Whereas a CSS framework generally focuses on providing a little bit of everything ("horizontal"): Bootstrap has a grid system (library), but also a button library, a modal popup library, etc, all bundled together in a one-stop shop for developer convenience.