When I implemented retrieval in our production system a few months ago, one of the most important benchmarks was cross-language retrieval (query in one language, documents in another), which is a common situation in large enterprises (headquarters + branches). I suspect their idea will perform poorly if the source language and the target language are too different from one another, like English and Hindi (grep often will not return anything).

Another requirement was keeping latency as low as possible (I managed to get < 5 seconds with 85%+ accuracy). Their approach seems to have very unpredictable latencies, sometimes up to thousands of seconds (may be fine for background tasks), and it scales poorly with corpus size.

Interesting research anyway, but I'd still stick with embedding/reranker-based retrieval because you do not waste time wandering around blindly each time, trying to find the minimal context to start from, which could have been found immediately with an index. Another issue is that research papers often implement subpar baselines for the approaches they compare against. When I was implementing retrieval, the straightforward implementation gave me 40% accuracy, and various tricks/parameter tuning pushed it to 85%+ without changing the overall architecture (took about a month of non-stop experimentation).