Is there really any evicende of it being a Phoenix egg? Unless it's retconned we have to take it for what it is. I just prefer to think that it doesn't matter at the end as PJean still had a part of Jean's soul and her memories/personality, and then Jean got them back on Inferno. The plan was for it go to her on the coccoon before she woke up, so it wouldn't even be like two different Jeans, that only happened cause Jean rejected DPS and then it went to Maddie.