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A commentum (from comminiscor) is, according to the Elementary Latin Dictionary:
an invention, fabrication, pretence, fiction, falsehood
At some point, a commentarium (and, I presume, its cognate forms) came to mean something much more similar to English "commentary/comment," e.g. Aquinas's In Aristotelis De Anima Commentarium.
How (and when) did commentum adopt this much more neutral meaning?
The Oxford English Dictionary entry for the English word comment says that Isidore's Etymologies (c. 600) uses commentum in the sense "a comment or interpretation".
I think it's necessary to distinguish between the word commentum and the word commentarium/commentarius when discussing the meaning and how it changed over time. I can't find any indication that commentarius in Classical Latin had any implication of deliberate falsehood or fiction: it seems to have just referred to a notebook or notes, as in Caesar's Commentarii de Bello Gallico.
I would guess that the usage of commentum in later texts like Isidore's was influenced by the meaning of commentarium and the verb commento(r).
