While I'm hoping that having the arresting agency remove the record of your arrest clears up everything, I wouldn't be surprised if DHS still treats the matter as a conviction and holds it as a reason to deny you entry.

You are unfortunately experiencing an inherent problem with the sealing, expungement or wiping of records--there are lots of records. If you get arrested by a local police department, they will have a record, as will the court that hears your case, as will the jail or other facility where you were booked (I know that not all of these terms and practices translate to Canadian law). The court may order your record expunged after you have completed probation, counseling, rehab or whatever, but either not everyone gets the word, or it drops between the cracks someplace. And every time an inquiry is made that results in a "hit" from an outfit that has a record that was supposed to have been sealed, the agency that made the inquiry will have the record, too.

This is what has happened with DHS, and the Canadian court has no jurisdiction to order the U.S. government to erase a record of conviction, even if the record wasn't supposed to have been reported in the first place. In this case, the DHS is (in theory) more concerned with your past conduct than with the legalities, and has complete discretion as to whether they retain the information, discard it, or ignore it.

I sincerely hope that you can resolve your problem, as it's kind of silly to regard an ancient minor indiscretion as a testament to a person's character. In fact, it speaks to your character that you told the truth in the first place. In this case, though, yours may be the good deed that shall not go unpunished.