These giant lemons look like they may have more citron ancestry in them. And not sure if this is at all applicable here but I will point out just a few of these traditional citron varieties are actually pomelo-citron hybrids (so that could possibly have implications about where these larger lemons were bred from).
I might mention the connection to Ponderossa lemon, which can get pretty big in size but isn't really the best tasting lemon out there.
I happen to have Giant citron and pomelo, and plan to be doing some hybridization experiments. Perhaps hybridizing the two together with yuzu I might get a large sized lemon with good flavor and more hardiness.