Here's the deal ... Ronson -- who's EP'ing the upcoming album attached to the movie -- has been teasing two final artists set to have songs on there (which weren't being announced until the final weeks). A few days ago, Billie Eilish revealed she was one of those two.

She confirmed it in an IG post, saying the track is going to be called "What Was I Made For?" ... adding that she's stoked to be part of the project, confirming her love of Barbie.

With BE being cemented as the penultimate addition to the soundtrack ... Twitter was rife with speculation about who could possibly be filling the final spot -- which reached a fever pitch when a screenshot of a track that was allegedly registered recently made the rounds.

The grab purports to show a song that's supposedly tied to the soundtrack ... one by Sam Smith and produced by Ronson -- with the actual "Barbie" soundtrack attached as well.

When the Internet saw this, they were bummed ... especially considering what Ronson had recently said about at least one of the mystery artists -- namely, "One of the f***ing greatest living artists around, but also had a very personal idiosyncratic tie to Barbie as well."

Indeed, the jokes were flying at Sam's expense ... as it appears fans were expecting a different artist to be unveiled. Well, it seems Ronson caught wind of all the chatter this morning -- because he hopped online to clear up who exactly he was talking about ... Billie.

He writes, "Hi. I love all these artists being thrown around. But was specifically referring to Billie." Now, MR didn't mention Sam's name specifically -- but does seem to be referencing all the conjecture that the final artist may be them ... so his comment here is interesting.