That difference is due to a systematic change in how arXiv generates URLs for submissions, starting a few years ago. No arXiv submissions today will have a URL with a category. Articles (each with a unique arXiv id YYMM.xxxxx) can have multiple topic tags; the article under discussion has two tags, one of which is "quant-ph". Cranky articles which cannot be refuted on technical grounds are often found on the "general" topic tags.

As an aside, since the above comment is essentially trying to infer the quality of the paper from metadata, it might be worth knowing that the author is a well-regarded quantum physicist with many highly cited papers.