For most recipes, the recipe is unlocked when the ingredient is obtained. But for honey blocks, it is unlocked when you obtain a honey block. i.e. there is no way to naturally learn this recipe in game without crafting it.

The advancements/recipes/decorations/honey_block.json file contains this condition as a criteria:

    "has_honey_block": {
      "trigger": "minecraft:inventory_changed",
      "conditions": {
        "items": [
          {
            "item": "minecraft:honey_block"
          }
        ]
      } 

It's probably meant to check for honey_bottle rather than honey_block