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The description of the spell is really quite clear, and applies to both versions of the spell (hostile target and friendly target, which includes the caster):

"The new form can be any beast whose number of Hit Dice is equal to or less than that of the target creature."

Note that it specically says, "beast" and not "creature".  Beast is a creature type, along with monstrosity, construct, aberration, humanoid, giant, undead, etc.

Note that these are D&D game terms, and thus referencing a dictionary definition is simply being silly.  For example, a giant is technically humanoid, in that it resembles a human: it stands upright on two legs, has a body, two arms, a head, etc.  For that matter, an iron golem is also a humanoid.  But neither of these things are "humanoids" in D&D terms, because "humanoid" in D&D has a specific meaning separate from the dictionary definition.

Just because something can be argued, doesn't mean it is at all correct.  People can argue pretty much anything.

The assumption in D&D should be that when a word is used that matches a keyword, the first default is to assume the word is referring to said keyword.  So, for example, if a feat says that it requires the use of a finesse weapon, you assume it is referring to the "finesse" keyword that certain weapons have as a trait.  You should not attempt to argue that it simply means you need to weild your weapon with a certain amount of finesse.  It is a game term.

I hope this puts the issue to rest, and I look forward to reading further arguments.