He wasn't a near unanimous first round talent. A LOT of the people who analyzed him on the field had him as a 2nd or 3rd round talent who acted like Caleb Williams in interviews. The people who lauded him as a 1st round talent were people close to Deion plus a lot of the mock drafts were not what teams SHOULD do but what they thought they WOULD do. Those are two different things.
Sanders was a 2nd or 3rd round talent who was not ready to start day 1 who had a disastrous interview process that made him fall to the 5th. Now yes once a decade you get a day 3 QB who becomes a starter. However it is FAR more likely he joins the 100 or so others who end up either career backups or just out of the league. The idea that he is a 1st round talent is one pushed by people close to Deion, many of whom work in the media, but he simply is not. He holds the ball too long, struggles at making NFL throws, and has a mediocre arm at best, all with limited mobility. The fact that Mel was trying to hype the idea of putting him in a dome kind of highlights that he needs advantages to succeed. The scouts view him as a product of Hunter in the same way Manziel was a product of Mike Evans.