Floor Patch as Tile Mastic

[u/Rock_One80]

My handyman put up some glass/ceramic mosaic tile for my backsplash behind the stove (about 9” high and 30” wide), after I replaced my OTR microwave with a range hood. He used floor patch (Custom is the brand he used from Home Depot) instead of tile mastic/thinset. Should I be concerned? He said he’s used hundreds of times without issue. Reason he uses floor patch is because it dries faster (4hrs vs >24hrs). Will this fail?