No, Fissure does not always hit a Pokémon that has used Dig. As you mentioned, Fissure has a 30% base accuracy chance. However, it hits Pokémon using Dig at its normal accuracy, even during dig’s semi- invulnerable turn. Additionally, It will always hit if the user is at least 70 levels higher than the target.
But, for OHKO moves, the base accuracy is not always the accuracy. It would be the base accuracy if the user was the same level as the target. The accuracy slightly varies based on the level difference. The formula for determining the accuracy of an OHKO move is: “acc. = (User's level - Target's level) + 30”. Finally, if the user is lower level than the target, it always misses.