I'm surprised the best song by the best pony has not made it into the game.
Yes, I am surprised the original is not in the game yet. Not the reprise from the end, and not a remix.
I am quite sure that this song is already in the game.
Also, as everypony knows, Derpy is best poni.
Ok, I can not argue with you, so long as it isn't Rainbow Dash, we are cool.
As for the song, sadly the reprise is in thee from the end of the episode, but not the original Diamond solo.
The part that is shown in the YouTube video which you posted (plus additions) is already in the game since end of last year. Either you are using a really old version or your setup is not complete.
I just got the game yesterday, and I only found the reprise...This is strange.
"only the resprise"
there is only one version (plus the instrumental) in the game. That is the full/extended/correct version. It contains what your video includes plus the reprise.
(https://i.imgur.com/44BAmHX.jpg)