They're not really being pessimistic, just telling you facts, and trying to help you not waste your money.
You see those raised spots on the obverse? that's the zinc underneath the copper rotting, that combined with the fact that there's no lustre whatsoever on this coin, clearly shows that it's been dug up.
I'm not going to try and stop you from sending it in, but it really is just going to be a waste of money for it to come back in a bodyback because of the exact reasons that have been stated here.
I suggest sending it in to ANACS, they're cheaper and arguably better at slabbing variety and error coins.
EDIT: here, if you look at some of our own users metal detecting finds, you'll see that a lot of them have this same grayed out look as your coin, some have some extra gunk on them because they haven't been circulated after being dug up like yours has. You'll also notice that a lot of the dimes have a copper look, which are also commonly mistaken as missing their nickel layer.:
https://goccf.com/t/187920https://goccf.com/t/179492https://goccf.com/t/179005https://goccf.com/t/179385https://goccf.com/t/172828https://goccf.com/t/175063https://goccf.com/t/178566https://goccf.com/t/175021