This theory operates on the assumption that Mordekaiser can't eat in his undead form.
1. The Noxian Empire invented the taco.
2. Morde died for the first time long before the Noxian Empire existed.
Conclusion: If Mordekaiser can only eat when alive, he has never eaten a taco.
Corollary: Sion has eaten more tacos than Mordekaiser has, and is angry all the time because he can't eat them anymore.