I take very detailed notes during these classes, but is anyone able explain this last point of the lecture. (See below)

What is actually said (taken directly from the Video Transcript tab):

With the exception of the plus equals, I didn't use an equal sign in here at all. All of the methods just change the list itself. They didn't create new versions of the list. This is the mutable aspect of lists. And it can definitely come back to bite you if you're not careful with it.

He's just reminding you that the operations shown in the video make permanent changes to the original list, which is possible because of lists being mutable.

The importance may become more clear if you compare these with the methods of strings, which leave the original alone and return new versions with the changes applied.

So what might "bite" is if you got these confused, and performed an operation thinking the original value would be preserved but it got modified instead.