This whole issue stems from the no_humans wiki using the word "humanoid" then defining it as "characters who appear identical to humans but are canonically not human".
Use this tag when no humans (or humanoids, e.g. human-looking homunculus, youkai, or the like) are visible in the picture.
The term "humanoid" commonly refers to the basic human shape - one head, two arms, stands on two legs etc. This implies that the tag is for posts with no characters who resemble the human shape (definition A). However, the examples that this wiki lists as humanoids are homunculi (artificially created beings meant to resemble humans) and youkai (spirits from Japanese folklore that are said to take human form). This definition is a lot stricter and implies that the tag is actually for any character that isn't of the human species (or a close fictional version, such as an elf), as it is essentially saying "don't tag no_humans based on lore" (definition B). The answer to this dilemma is whether we want the tag to use definition A or definition B.
If we choose definition A, then the tag would be intuitive - no_humans means no_humans (within reason). However, as stated in the OP the problem of being unable to look for posts with zero human-shaped characters will remain. This is something I can imagine a decent amount of users would enjoy being able to search for.
If we choose definition B, then the tag should be renamed to no_humanoids and the wiki should be rephrased in order to prevent any confusion. However, users will lack the ability to blacklist characters that aren't human but are still humanoid-shaped - I imagine a large amount of users would like to be able to easily blacklist non-human characters such as furries, androids, demons, monster girls/boys etc. as they're not to everyone's tastes.
A solution that would solve both issues would be to use both: no_humans for definition A and no_humanoids for definition B, with no_humanoids implying no_humans. That way users are able to blacklist non-human humanoid-shaped characters using no_humans, while also retaining the ability to search for posts that lack any humanoid-shaped characters at all. This is the choice that I'm in favor of; feedback would be appreciated.