Some of the same people developed both. Pretty sure Jyrki Alakuijala for example led the development of lossless mode for both WebP and JPEG-XL.
I designed WebP lossless alone. The rest of the WebP folks added a RIFF header and an artificial size limitation (16383x16383) to match with the size limitation of lossy WebP.
In JPEG XL I believe I had more influence on the lossy ("VarDCT") format than anyone else, but stayed relatively far from the lossless part (except the delta palette, two predictors, 2d lz-codes, and a few other smaller things). I believe Jon and Luca had most influence there.