Just change the c:tools/knives to c:tools/knife

If you look at the Tool subgroup, Neoforge uses singular naming for the tools, not plural.
https://github.com/neoforged/NeoForge/blob/1.21.x/src/generated/resources/data/c/tags/item/tools.json

This way, the fillet knife will comply with the tag system and be compatible with other knife recipes.
Especially now that Farmer's Delight released to 1.21