That is correct. I can not combine enchanted books and tridents, with trident specific enchantments. Or crossbows with big pls with crossbow specific enchants. And I suspect, but can’t attest to, shields and books with shield specific enchants.
I can however put unbreaking or mending books on them.
And combine tridents with trident specific mods that were enchanted using vanilla methods with tridents that have unbreaking and or mending on them.
But I can not combine tridents that have trident specific mods on separate tridents. Regardless of weather or not the enchants conflict.
For example riptide and Impaling