It's obvious that he was a late addition, but that still doesn't say anything about delaying the game, and he was apparently added quickly.
If I had to make a guess, I'm pretty sure Sakurai had some documents prepared to show Sega whenever he first began negotiating Sonic's inclusion. Maybe he even had an early playable version to show them (as he would have to convince them he could create a good portrayal of the character). And he was probably planned early on too, but Sakurai simply didn't have the rights to officially add him until later on (because he was already considered for Melee, so the thought had to have been there).
So I think there must be some confusion regarding this. We know that he had difficulty making the negotiations with Sega. But adding a character to the roster constitutes having the rights to the character, as well as the playable build and franchise representation in place of that character. So I think some work on Sonic had been done before 2007, but it just took Sakurai a while to convince Sega, and he needed material to show off. He wasn't given the A-OK until later on, but he already had work done on Sonic and his representation at that point. I don't believe Sonic was part of the initial roster plan, but I don't think he was added so late that it caused a delay. That's not a thing Sakurai would do, considering Sonic and Snake didn't make it into Melee, but were considered.