Honestly, unless they are high-quality show bred white Plymouth Rocks, ear lobe color isn't going to mean much. It's shape and size that will mean more. Post a recent picture of them and I can give you my best guess.
As a side note, if you were meaning the egg at the bottom left of the picture you posted, it looked more like a light brown egg than a white egg to me. This is a plausible outcome from outcrossing white Rocks to white Leghorns in order to up production, which is what was done in some hatchery Plymouth Rock stock from what I understand.
And yes, sexlinks are a hybrid. Mutts are usually accidental crosses or crosses made on a whim. Hybrids are bred for a specific outcome, in this case being sexable upon hatch.
Pip, your knowledge base is impressive. You must read every bit of chicken litterateur available to you.
Good going kiddo!