The “key” of an m.2 defines what the pins mean, basically what signal they carry (PCIe, USB, …). There’s a nice table here, if you scroll down a bit. Some are extensions to others, and are pin compatible (meaning the things they have in common are on the same pins).
A key and E key are very similar, while E just provides a few more interfaces, but importantly A doesn’t provide anything the E doesn’t. So any card that can work in A can also work in E. This is why A+E is so common: they don’t require the Mainboard to provide E, only A, but both will work so both notches are present.
That story is genuinely hilarious. And from the judges summary judgement it really does sound like the license holder of the disputed songs did some legal juggling just to be able to play the victim and sue Spotify. What an odd business plan…