The coins used in the Roman Empire often remained in circulation
It's unclear what the argument means by "the coins were dated to 375 A.D." If it means, like modern day coins, "the coins were marked with the date 375 A.D. to signify the year they were minted", then this answer would mean that a coin made in 375 A.D. would still probably have been around 50 years later, so maybe someone in 425 A.D. still had this coin. If it means, we did radio-carbon dating to see how old this coin was and we could tell it was as old as 375 A.D., then that doesn't tell us exactly when it was minted, so this answer doesn't have any direct impact. In either case, this answer suggests that a coin that originated in 375 A.D. might still have been in someone's pocket 30-50 years after that, when they decided to chuck it in the well. If they didn't throw the coin into the well until even later, like 400 A.D., then that kind of strengthens the conclusion. It sounds like we could say, "no earlier than 375 A.D.? This coin probably wasn't thrown in until 400 A.D., so the kitchen implements are probably younger than 400 A.D.", which would agree with the conclusion. But this answer is just a very weak, general statement and so it's hard to draw any meaningful inference about whether this tells us anything about the coins in the well. We already know that one of them is at least as recent as 375 A.D. We don't really get any new strengthening value by saying, "Actually one of them is at least as recent as 390 A.D."