“hydrolysate” is not just a posh word for digestible protein. It usually means a material where proteins or other polymers have actually been broken down by hydrolysis.
A proper hydrolysate should have measurable breakdown. For a protein hydrolysate that usually means things like degree of hydrolysis, soluble nitrogen, free amino acids and the amount/size range of small peptides. In other words, it is not just “old”, “boiled”, “more digestible” or “a bit fermented”. There should be a controlled breakdown of the raw material. Either via enzymes, acids, fermentation or other methods.
Uncontrolled fermentation is different. It may create attractive compounds, and it may partly hydrolyse proteins or carbohydrates, but the result will not be consistent unless the process is controlled. The final product will depend on which bacteria or enzymes dominate, the pH, salt level, temperature, oxygen exposure and how long it is left.
That is also why homemade ferments can be risky. If you don't have control, the wrong microbes might get involved and you may be growing potentially dangerous bacteria.
In addition to botulism, you can add Salmonella, staphylococs, listeria and several others.
Some of these are mainly a risk to the person handling the bait. Others may also matter for fish health or water quality.