Or, if you prefer it watered down:
(A strong case could be made that) Haiku is (in significant ways) the opposite of poetry.
Look, I know that we are unlikely to find a definition of haiku that doesn't start with the words, "Haiku is a short poem that…" And I know that many, if not most, haiku enthusiasts also have an interest in the wider world of poetry as writers and as readers. I don't. I may joke that I am allergic to poetry; but the fact that my love of most things haiku and my loathing of most things poetry seem to be connected intrigues me: perhaps it is not the case that haiku is, as everyone else seems to take for granted, simply a short form of poetry.
Clatter about in the poetry toolbox that you may have had since schooldays, and you will find:
and so on
Most of these have little or no place in haiku. This not to do with "rules" as such - rather, it is that experience has shown that, to the extent that they are noticeable, they have a disruptive and deleterious effect. So, If the features that we think of as being most prominent in (western) poetry are required to be - to all intents and purposes - absent from haiku, how does it make sense to categorise H as being simply a subset of P? At this stage in the argument it seems more as though the values of H and P are in inverse proportion to one another. Specifically, the greater the amount of P injected into H, the less effective that H is likely to be. Haiku is categorically not, therefore, "Poetry, only smaller."
There are other aspects in which Haiku and Poetry seem to be opposites. But it is more helpful to think of these as tendencies, inclinations, or impulses, rather than fixed. Here, I like to imagine a kind of virtual graphic equaliser, where you can move the slider upwards towards Poetry, or downwards towards Haiku, it being for the writer to determine what balance "sounds" best.
The Poetic tendency, you could say, pushes the slider upwards towards Self-expression; towards dramatisation and self-dramatisation. The Haiku tendency is in the opposite direction, towards restraint, stating things as they are, and playing down the Self: the "I" rarely taking centre stage.
Again, the Poetic inclination might be to tell, explain, propound, to over-state. The Haiku inclination would be to under-play, to hint, to suggest; emotion, for example, rarely being stated outright.
The Poetic impulse would be to draw attention to the words themselves, the poet garnering applause for how well they train their words, images and poetic devices to perform tricks. The Haiku impulse is the very opposite; if overt poetics intrude, the haiku is spoiled.
The direction of Poetry might be towards elaboration, adornment, complexity, image piled upon image. And here again the direction of Haiku is the very opposite.
It seems to me that a haiku is only a poem in the sense that it is a grouping of words that can hang, self-contained, framed in the white of a page. And we don't have any other word in the language to describe that sort of thing. In most other respects the qualities of Haiku and the qualities of Poetry seem to work against each other. i.e. Haiku really is the opposite of poetry.