It's too hard because the variations you could add to it (multi threading) that add enough depth to make it hard make it too hard, in my opinion. If you look at the implementation I linked in my previous comment, it's fully lock-free, which is pretty unreasonable to expect from anyone who isn't already familiar with lock free concurrency. On the other hand the version with a lock is basically identical to the single thread version. Asking for the two-lock queue is also a bad interview question because it's not something you'd reasonably expect someone to derive in an interview.

The other examples given for fleshing it out are all pretty similar; if a candidate can do one, chances are they can do the others too. If you want to get a decent signal if candidate skill, you have to ask a question easy enough that any candidate you'd accept can answer it, then incrementally add difficulty until you've given the candidate a chance to show off the limit of their abilities (at least as applied to your question).

Otherwise you ask a too-easy question which everyone nails, then make it way too hard and everyone fails. Or you ask a too-easy question and follow it up with additional enhancements that don't actually add much difficulty, and again all the candidates look similar. That's just my experience; the author seems pleased with the question so maybe they're getting good signal out of it.