Wiktionary says on PIE -h₃onh₂-:
Latin: -iō (from *-i-h₃onh₂-) (e.g. legiō (“group of selected people”))
Latin: -ō (e.g. Nāsō (“having a conspicuous nose”), poss. Iūnō (“having heavenly authority”))
Latin: -tiō (from *-ti-h₃onh₂-)
dictātiō (“a dictating, dictation”), from dictātum, supine of dictō (“I dictate”)
quadripartītiō (“a division into four parts”), from quadripartītum, supine of quadripartiō (“I divide in four parts”)
You see, this article does not say that dictātiō is from dictō, but it says it is from dictātum, supine of dictō.
There are two supines, I (first) and II (second). They are originally the accusative and ablative forms of a verbal noun in the fourth declension, respectively.
non-finite forms - participles - passive - perfect : dictātus
verbal nouns - supine
- accusative : dictātum
- ablative : dictātū
They have common part dictāt, (but most of them has common part dictātu), I can conclude that maybe supine of dictō has stem dictāt, and dictātiō is dictāt + iō.
Most forms of "dicto" have common part "dicta", so I can conclude that maybe dictātiō is dictā + t + i + ō , where t is passive perfect participle suffix and ō makes agent noun from verb according to https://en.wiktionary.org/wiki/-o#Latin . so, "i" should be a suffix that makes a verb from a noun.
Was there really "i" suffix that makes verb from a noun?
Or maybe really dictātiō is not from dictātum, but directly from dictō (infinitive dictāre) as dictā + ti + ō, in that case what is meaning of "ti" suffix that does not consist of t + i?