So this sentence doesn't follow V-2 rule?
Correct. Not entirely sure why, though...
It's because of the bare if I remember some podcast correctly. "Small words" can be injected and stirr the rules.
I think it's for emphasis on bare.
When do you use kun and when bare?
"Kun" is like the word, "exclusively," i.e., to the exclusion of all/everyone else.
"Jeg advarer kun deg," would be along the lines of, I'm warning you, and only you," or, "I'm warning you, exclusively."