So I have two battle.net accounts. One of them is ancient from a childhood playing Blizzard games on my PC, and the other is a newer one that I created since I had originally lost access to that childhood account. So I went to transfer over the Destiny 2 save to my steam account and apparently picked the wrong account. I'm still not completely certain which one is actually correct, as battle.net doesn't show the game for any of my accounts anymore. I had no reason to believe I had picked the wrong account as the PC Move was completely okay with transferring over data from an account that had no Destiny 2 and even showed licenses. I didn't even need to log in to my Battle.net, it was automatic, how convenient. After the initial bugs and failures to migrate, I was excited to finally be able to transfer over my character. Over on steam, no characters, no hint of anything happening, no forsaken or DLC. The account I apparently transferred from had nothing because it didn't even own the game, never had it installed. Now it just continues telling me that the steam account is already in use by another account which is simply not true - there is no account on my profile. I'm not going to create a new steam account for this single game, I feel rather screwed over by a system that was buggy and rushed. Bungie's twitter is claiming the PC Move issues are solved, but you never actually solved a bunch of problems that people had and instead direct people to create new accounts. Workarounds aren't solutions.
It sounds like your browser auto-filled your info, when you went to setup your PC Move info, causing you to link the incorrect Battle.net account. Regardless of the fact that no Destiny data existed on that account, which would have been shown to you during the setup, the system still went through the process of transferring data, as you had authorized it to do. Now that your Steam account received a data transfer, it is no longer eligible to receive further transfers. There is no option other than to create a new Steam account, if you want to bring the Battle.net data over from your proper account. If Bungie was able to offer a different solution to this, they would have long since done so.