They used a technique for water segmentation described in an earlier paper [1]. The validation dataset for that method goes back to 1992...

"All applied images have 30-m spatial resolution and were acquired during October 1, 1992 to October 31, 2018. The period was set with regard to the availability of the validation dataset"

[1] Yao, J. Wang, C. Wang, J.-F. Crétaux, Constructing long-term high-frequency time series of global lake and reservoir areas using Landsat imagery. Remote Sens. Environ.232, 111210 (2019).