Egg shells and groceries trash as fertilizer yes, but this is first time I heard any leftover bones/bone dust can be used to enrich soil nutrition.
For those curious there are three primary ingredients in fertilizers:
Nitrogen - Compost (Rotting organic matter, like food), manure and from planting legumes in the area as it naturally bring nitrogen to the soil.
Phosphorus - Bone Meal (Ground up bones), Seashell Meal, and Eggshell Meal.
Potassium - Compost, Manure, Ash (From wood/plant; Also known as potash, but different from potash made today) and for their location they can also use Kelp Meal (Dried kelp/seaweed ground up).
I feel like I have it practically memorized from how frequent this pops up in isekai stories. Though I wonder how many address things like overuse of fertilizers. Like another person mentioned, there's a reason stuff like bone meal is often used for crafting recipes in games.