I think you missed a point, they weren't talking about the interaction of Spellshield with your own Mystic Shot. What they were saying is your can fizzle Cataclysm by killing your own unit, but you can't fizzle Cataclysm by using Spellshield, which feels a bit counterintuitive
Cataclysm requires all targets to be present in order to cast. If you kill your own unit, this condition is not met and thus the spell fizzles.
Spellshield doesn't fizzle Cataclysm because the target is still there. It does do something (it pops the Spellshield), so having the spell fizzle in this case would also feel a bit counterintuitive.
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