Tested with FAPI 0.115.1+1.21.1.

For mod compat reasons, I have two types of recipe, one that should load if the c:honey tag is populated and one as a fallback.

I noticed that even without any mod adding such a fluid, my load condition that read

"fabric:load_conditions": [
    {
      "condition": "fabric:tags_populated",
      "registry": "minecraft:fluid",
      "values": [
        "c:honey"
      ]
    }
  ]

evaluated to true, stating that the tag exists, even if the tag was completely empty. The check in the load condition merely checks, if that tag is in the list of existing tags, it does not, however, check, if the tag is actually populated.

Based on this documentation, this behavior is intended.