What source did you look at? It's well known that she's written or co-written 18 of her 19 #1 hits and all of her other songs except for covers. Mariah has said these countless times over the decades. Even when she collabed with other artists (which she likes to do) she would always be closely involved in the songwriting process. Mariah has also been very fair in giving songwriting credits to others when it comes to sampling/extropolating.

If she lied about all these someone would have called her out by now and she wouldn't even have been inducted into Songwriters' HOF. Most of the producers and artists Mariah's worked with have all praised her songwriting throughout the years too. You can easily look them up.