In Gen 4, there are 442 moves that Metronome can call, so there was a 1/442 chance of getting Sheer Cold, and then the same chance of getting Fissure after that. We could change this to a 3/442 chance for the first and a 4/442 chance for the second to count the other OHKO moves that could have been pulled here for the same outcome (only 3 for the first because Fissure doesn’t affect Zubat), but you pointed out how both moves would have been super effective if they were regular attacking moves, so I’m going to keep them as a 1/442 chance, each.
OHKO moves use a unique accuracy formula that is unaffected by accuracy and evasion modifiers (including Gravity). The formula is
(user_level - target_level + 30) * 1%. Dawn’s Clefairy was level 25 and used Sheer Cold against a level 24 Zubat, giving it an accuracy of 31%. Your Togetic was level 28 and used Fissure against a level 26 Stunky, giving it an accuracy of 32%.
Here’s our final equation:
1/442 * 31/100 * 1/442 * 32/100 = 62/30,525,625, which is approximately 1/492,349, or a 0.0002031080445% chance.