Arcane magic as the magic of Order makes more sense if one thinks in terms of Order and Disorder in their contexts as verbs. Chaos is a relative state of something, while Order and Disorder - in the context of “Ordering and Disordering” - is the nature of a change taking place rather than either being a static endpoint.
To Order is to organize and build things up. To Disorder is to disorganize and tear things down. Either will be relatively chaotic when there’s no purpose or intent (or at the very least, not observably) directing them, but neither is truly fundamentally chaotic in its nature or there would be no consistency to how any spells work and magic would be impossible. Each has its own internal rules that govern how they function, which is why usable magic with reproducible outcomes exists at all, as learning to use fel or arcane magic amounts to learning proven processes that shape the energies certain ways any time they’re done correctly.
Hence why arcane energies unleashed without purpose start manifesting organized crystalline structures and geometric matrices in their surroundings, attempting to “build systems” in the absence of direction, while Fel energies unleashed without purpose manifest as volatile green fire or sludge that corrode and destabilize their surroundings, attempting to “break systems down.”
Arcane exists with Fel in the Twisting Nether because rather than just a “static” vacuum state of Order already being absent, Disorder is the literal function of “Ordering being undone.” So that manifests as a plane in which the raging energy of Disorder is perpetually breaking down the emerging energy of Order. And to varying degrees as well; Chronicle established that while the worlds upon which demons live exist in the Twisting Nether, it’s at the “periphery” where it exists “closest” to the Great Dark and the natures of both planes bleed across to overlap, meaning tangible things like planets and the demons inhabiting them can exist in an otherwise ethereal plane because the proximity of the Great Dark allows Order to take root and stabilize more firmly there against the Disorder of the Fel than it can further within the depths of the Nether.
(Consequently, it’s feasible that Outland may technically be submerged further into the Twisting Nether than even worlds occupied by the demons, as demonic homeworlds and the planets that get dragged into that plane by the Burning Legion’s corruption have continued to exist - albeit worse for wear - for ages, while Outland has been gradually disintegrating and crumbling from its edges through exposure to that realm ever since it was sent there.)
Long story short, to some degree “Order and Disorder” in WarCraft’s context are more like the dynamic processes of “creation and destruction of systems” rather than the inverse endpoints of something being either orderly or chaotic. Unlike Light and Void (with the conceptual “endpoints” of existence or nonexistence), or Life and Death (i.e. alive or dead), Order and Disorder are more like the manifest sliding scale of creation and destruction. To frame it with the actions of beings associated with Order and Disorder, the Titans’ nature is to build and make, while demons’ nature is to smash and ruin. As stated above, this same basic nature also manifests in the unsupervised spontaneous behaviors of Arcane and Fel.