On the surface you might be thinking this is an unlikely duo. Then you have to remember the 2001 Grammy's where they performed Eminem's hit song, Stan together. They have remained friends over the years.
Elton John did a Detroit themed episode of his Beats 1 show so of course he invited his friend Eminem. They talked about Em's BET freestyle, Elton John helping Eminem quit drugs and his beef with Machine Gun Kelly.
Elton John admitted to being a fan of Machine Gun Kelly's, Killshot, the diss track toward Eminem (that had to be awkward)
About the BET freestyle, Eminem was suppose to perform it live on the BET Awards but they were afraid with people recording things on their cell phones that it would be released in pieces and not in it's complete form before the show aired so they opted for the version we now know of him performing in a garage.
Elton John commended Eminem for deciding to stop using homophobic slurs.
More details HERE