I can only surmise that the lime tree scion did indeed die and the unnamed citrus rootstock has survived and is now fruiting.

Thanks. It's quite strange. :)

We grow both the Bearrs/Tahitian/Persian lime and the Mexican/Key lime. Fruit can be picked from both of these when the green fruit are full-sized. If you leave them on the tree, fruit of both will eventually turn yellow. Just because the fruit is yellow does NOT mean that it is a lemon; it is a yellow lime.