"The Menagerie" is similar to a clip show from a production standpoint, but really kind of the inverse of one from a story standpoint, in that it consists primarily of the original episode and secondarily of "The Envelope" created to expand it and contextualize it within the series. That's the reason it uses the same (final) title as the pilot -- from a production standpoint, it was the pilot with some additions. (Indeed, the actual physical film master of the pilot was cut up and combined with the new footage to create the master of the 2-parter, which is why we didn't have a complete color copy of the pilot for so long.) It's similar to how a film like, say, John Carpenter's student film Dark Star had multiple new sequences filmed and edited in to expand it to feature length for theatrical release. Or like how Gojira had a bunch of new stuff with Raymond Burr shot and edited in, and a lot of the original content cut out, to turn into Godzilla, King of the Monsters!.