Because, your parent seems to be saying the former (that we could comprehend that it exists, but wouldn't understand its motives, etc.).
But that's a non-statement. I don't think there's much question that we'd be able to perceive anything that our senses can detect. And, an ant can grasp that an object exists (and, for instance, that it needs to traverse it), even if it understands nothing about its purpose, who built it, etc.
So, it would seem that the only sensible intent in this statement by Musk (and the interpretation that you, but not your parent, seem to support) is that we would have a better understanding of this alien super-intelligence than an ant would of us, simply because we'd crossed some imaginary and arbitrary intelligence threshold, defined by Musk.
It's a comforting (and very human-centric) thought, but I don't think it's true. Considering "infinite intelligence", there is some point along the continuum wherein our relative intelligence to a super-intelligence is akin to an ant's to ours. So, I don't see any reason why we'd better understand the motives, etc. of such a super-intelligence than an ant would ours. To make that statement is to define the bounds of the super-intelligence based on ourselves and our own boundaries, which is to say that it involves completely circular reasoning.
And here is the actual paragraph:
"One topic I disagreed with him on is the nature of consciousness. I think of consciousness as a smooth spectrum. To me, what we experience as consciousness is just what it feels like to be human-level intelligent. We’re smarter, and “more conscious” than an ape, who is more conscious than a chicken, etc. And an alien much smarter than us would be to us as we are to an ape (or an ant) in every way. We talked about this, and Musk seemed convinced that human-level consciousness is a black-and-white thing—that it’s like a switch that flips on at some point in the evolutionary process and that no other animals share. He doesn’t buy the “ants : humans :: humans : [a much smarter extra-terrestrial]” thing, believing that humans are weak computers and that something smarter than humans would just be a stronger computer, not something so beyond us we couldn’t even fathom its existence."