Learn Finnish: Tuli kommentoitua tuota julkaisua vahingossa. - I accidentally commented on that post.

The inadvertent action structure (tuli + partitive passive past participle) is inherently impersonal. While you can specify the person using the adessive case (e.g., minulla tuli kommentoitua for "I accidentally commented"), it is very often left out in natural speech when the context makes it obvious who performed the action.

How exactly is the form kommentoitua built from the base verb kommentoida?

This structure requires the passive past participle in the partitive case. First, find the passive past participle of kommentoida (a type 2 verb), which is kommentoitu (commented). Then, add the partitive ending -a to get kommentoitua.

Is tuota julkaisua in the partitive case because of the "tuli tehtyä" (inadvertent action) structure?

No, the structure itself doesn't force the object into the partitive here. It is in the partitive because the verb kommentoida is a partitive verb, meaning it always requires its object to be in the partitive case, regardless of the grammatical structure around it.

Since the "tuli" structure already implies an accidental action, isn't adding the word vahingossa (accidentally) redundant?

Technically yes, it is a bit redundant! However, it is extremely common in everyday Finnish to use both together. The structure tuli kommentoitua translates to "happened to comment", and adding vahingossa just strongly emphasizes that it was a pure mistake.