It's not about unblockable, it's about the number of decision points the opponent has throughout the setup.
Think of it as a flowchart. There's a lot of potential decision points for the opponent to make that gets them out of the setup and results in Raiden getting punished. To make your sequence work, they'd need to be hit by the D2 KB and opt not to breakaway, then fail to block & punish a raw B2, then simply let the Raiden player walk in and get hit by another raw B2, and again opt to not breakaway from the B2 KB.
As a hail mary play it's fine. It's just that it requires you to have some very specific resources still available, and for the opponent to make 3-4 bad reads in a row. Against someone who doesn't know the matchup, or their options, or is a lower level player it'd probably work every once in awhile. I think a higher level player would likely blow this up 90% of the time.