Why is it wrong to treat «الذين يستكبرون عن عبادتي» as unrelated to the warning?

Defining the Subject by a Relative Pronoun: Purposes and Rhetorical Applications

• Because the verse is only a grammatical exercise
• Because arrogance is meaningfully connected to the warning
• Because there is no predicate
• Because «يستكبرون» is a demonstrative