Yes I'd expect a dict-based approach to do better here. That's probably how it should be done. But --long is compelling for me because using it requires almost no effort, it's still very fast, and yet it can dramatically improve compression ratio.
Have any of you tested it?
PS: what the author implicitly suggests cannot be replaced with zstd tweaks. It'll be interesting to look at the file in imhex- especially if I can find an existing Pattern File.
Or another way to look at this is that the total bytes saved of the dictionary will plateau. So your dictionary may save 50% of the first MB, 10% of the next MB and 5% of the rest of the first 10MB. It matters a lot if you are compressing 2MB of data (7% savings!) but not so much if you are compressing 1GB (<1%).