Presumably that is only one of two purposes, with the other being to test your ability to efficiently read, understand, and edit low level code that you didn't write. I imagine you'd regularly run into raw PTX if you worked for them in the relevant capacity.
And perhaps a third purpose is to use the simulator to test your ability to reason about hardware that you are only just getting familiar with.
I would assume that anyone optimizing kernels at Anthropic has full documentation and specs for what they are working on, as well as a personal butler attending to their every need. This is big money work - every 1% performance improvement must translate to millions of cost savings.
Maybe they specified the challenge in this half-assed way to deliberately test those sorts of skills (even if irrelevant to the job), or maybe it was just lazily put together.
The other thing to note is that if you look at what the reference_kernel() is actually doing, it really looks like a somewhat arbitrary synthetic task (hashes, wraparound), so any accurate task specification would really need to be a "line by line" description of the steps, at which point you may as well just say "here's some code - do this".
In a fast-paced domain such as this one, and especially wrt the (global) competitiveness, development/leadership process is most likely chaotic and "best" practices that we would normally find in other lower-paced companies cannot be followed here. I think that by underspecifiying the assignment they wanted to test the ability of a candidate to fit into such environment, apart from the obvious reason and which is to filter out not enough motivated candidates.
And perhaps a third purpose is to use the simulator to test your ability to reason about hardware that you are only just getting familiar with.