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I love autism like this.
The Jutlin is a fine choice for Tridax. He was assigned to Nynrah, which had fe-matoran. We know Toa of Iron was one of the types hunted by the makuta due to their element being strong against making armor.
It was also stated there were no toa on Nynrah.
It makes sense that Tridax killed any toa of iron that protected the fe-matoran, and the best mask to use against such a foe is the kanohi that makes objects rust.