I noticed you used his spread, but seemed to use a different nature (be it Timid, Calm, Bold, whatever, it wasn't Modest). If you use Modest, Volt Switch jumps up to 50% chance to OHKO with that spread.
252+ SpA Rotom-H Volt Switch vs. 88 HP / 0 SpD (custom): 294-348 (92.16 - 109.09%) -- 50%...