> It just means you're financially secure enough not to have to choose between food, clothing, shelter, and healthcare with every paycheck, and to be able to save up a buffer in case of emergency.

I would say that more accurately it means that a portion of your income is made passively (which that buffer provides) alongside having a working income. The working-class, as the name implies, make all of their income by working, and the upper-class make all of their income passively.

But the fun thing about middle-class is that it has no generally accepted definition, so it can mean whatever you want it to.