The accessibility statement refers to things working for everyone - or as w3 puts it
The Web is fundamentally designed to work for all people, whatever their hardware, software, language, culture, location, or physical or mental ability.
The physical and mental bits are by far the most important things in that statement, all the rest is just compatibility IMO.
So what bootstrap docs is saying is there may be issues with things like screen readers for the visually impaired being able to read the content inside the carousel and such.
As far as compatibility goes, one of the main features of Bootstrap is compatibility. They’ve done most of the browser testing and fixes for you. So don’t worry, for this exercise its perfectly acceptable to use the carousel if you want to and it should work just fine.
At some point in your learning you will need to get familiar with accessibility standards but there’s loads of info on the web and every book I have seems to touch on it too.