Since you mentioned it, all the options for exporting WebP images in GIMP come from Google's WebP code library. The "Source Type" is used in the library's WebPConfigPreset () function - basically, it sets some default quality values for export. All we do in GIMP is set it in Google's library to whatever the user selects - beyond that, it's not something we have control over.
Discussion of: How to Make WebP Images Less Blurry
Hi! I'm one of the volunteer developers for GIMP. Glad the SharpYUV feature is helpful (it was added by user request in https://gitlab.gnome.org/GNOME/gimp/-/issues/8110)
Since you mentioned it, all the options for exporting WebP images in GIMP come from Google's WebP code library. The "Source Type" is used in the library's WebPConfigPreset () function - basically, it sets some default quality values for export. All we do in GIMP is set it in Google's library to whatever the user selects - beyond that, it's not something we have control over.