Totally legit, though vcoins is probably a more reliable site since fakes do exist (modern and contemporary).

The Roman span was enormous and long lived. They also heavily used coinage and had traveling mints. The coinage was eventually debased but there's ~500 years to cover and their "pennies" are everywhere. There's also not too much of a demand for them compared to the amount produced.

Right now US coinage is valuable so we hang onto it but if everyone just tossed their coins tomorrow, there would be plenty for future generations to have a few a piece.

Roman empire was very extensive (comparable to modern EU in size), fairly monetized, spanned centuries, and modern civilization digs a lot in the same places (if only for construction purposes), so the total amount of Roman coins unearthed is huge.