
Daniel P. answered 10/20/20
Versatile Tutor with Advanced Expertise in English
I would say that your teacher *could* present a case in which the second sentence is structurally parallel, but it's certainly not as strong or rhetorically obvious.
The second sentence is parallel at the level of grammatical parts, but not at the level of phrases.
"I am king in my own land" is structured as: [subject (I)] [verb (am)] [subjective complement (king)] [prepositional phrase (in my own land)]
"will never become vassal of a mortal like myself" is structured as: [verb (will never become)] [subjective complement (vassal)] [prepositional phrase (of a mortal like myself)].
It's a quite messy parallelism because the modifiers of "land" come before it ("my own") and the modifiers of "mortal" come after ("like myself"), so I can see why you disagree with the teacher!