Why not? Even without LLMs it is technically feasible to build custom database engine that performs much better than general database kernels. And we see this happening all the time, with timeseries, BLOBs, documents, OLTP, OLAP, logging etc.

The catch is obviously that the development is way too expensive and that it takes a lot of technical capability which isn't really all that common. The novelty which this paper presents is that these two barriers might have come to an end - we can use LLMs and agents to build custom database engines for ourselves™ and our™ specific workloads, very quickly and for a tiny fraction of development price.

If you look into the results, you will see that they are able to execute 5x TPC-H queries in ~200ms (total). The dataset is not large it is rather small (10GB) but nonetheless, you wouldn't be able to run 5 queries in such a small amount of time if you had to analyze the workload, generate the code, build indices, start the agents/engine and retrieve the results. I didn't read the whole paper but this is why I think your understanding is wrong.