She was certainly a fine guitarist, but Wikipedia says you were right first time, it was a session player called Chip Young.

Dolly wrote the lick herself, and when she was in the studio with the session players, they wanted her to show them how to play it [0].

She ended up having Chip Young lay it down on the record since her long fingernails made it difficult to nail consistently. This is also why if you watch videos of her playing guitar she plays in open tunings [1].

0: https://www.theguardian.com/music/2024/oct/31/dolly-parton-r...

1: https://www.guitarworld.com/features/how-does-dolly-parton-p...