The existence of alternative economic growth models outside of the Solow-Swan framework is a foundational textbook fact in macroeconomics (such as endogenous growth theory, AK models, and Ramsey-Cass-Koopmans models), requiring no citation.
The claim states that economic growth models exist outside the Solow-Swan model. This is an indisputable baseline fact in economics; numerous alternative frameworks (endogenous growth theory, Harrod-Domar, overlapping generations models, etc.) are standard material in any economics curriculum. Therefore, this is common knowledge and requires no citation.