That's right, Trek or Treat. But if you need serious...
Sylvia and Korob were so alien that humanoid mind and humanoid life were totally foreign to them. To understand and/or control, they attempted to probe the conscious minds of crew and instead reached the unconscious mind, where they were unable to determine what is human reality and what is human imagination.
They probed the basic primal unconscious human mind, elemental fear, which they manifest in symbols of horror and terror (skeletons, torture, ghosts, etc.)
Like most ST, it's not really a bad idea, maybe just failing in execution. And it's been seen many times in ST... aliens who seek to understand mankind and latch on to one aspect by which to judge us (and make an eps)
I like that interpretation. And an extension of that blurring of reality and imagination was the death of Korob by the cat, because... after the scepter is destroyed we see both marionette aliens withering (rather than one dead and the other about to die). So even his death was an attempt to elicit a response from the landing party.
About the only real "major fail" in the episode for me was the depiction of the aliens. Seriously, they could have come up with something better rather than slow motion puppets with visible strings