Is the methodology behind the estimated SLEDAI scores validated?

Yes. The machine-learning based methodology for estimating SLEDAI scores for patients is fully validated and published (Alves P, Bandaria J, Leavy MB, et al. Validation of a machine learning approach to estimate Systemic Lupus Erythematosus Disease Activity Index score categories and application in a real-world dataset. RMD Open. 2021 May;7(2):e001586.