Maybe, but the Sri Lankan Sangha follows the Pali in all questions of Vinaya. I suspect this is related to the commentarial explanation, or perhaps this is more of a cultural phenomenon.
The word vacanapatha is actually quite rare in the EBTs. Apart from the garudhammas, it occurs only in a few contexts, all of which refer to unpleasant kinds of speech:
… duruttānaṃ durāgatānaṃ vacanapathānaṃ …
… vacanapatha that is rude and unwelcome … (e.g. MN 2)
Passatha no tumhe, bhikkhave, taṃ vacanapathaṃ, aṇuṃ vā thūlaṃ vā, yaṃ tumhe nādhivāseyyāthā’’ti?
Monks, do you see any vacanapatha, subtle or gross, that you would not be able to endure? (MN21 and MN 103)
… yāva na amanāpā vacanapathā phusanti.
… as long as they do not experience unpleasant vacanapatha. (MN 21)
MN 21 also speaks of five _vacanapatha_s, not all of which are unpleasant. Yet the emphasis in this sutta is on how to deal with unpleasant speech in a constructive way, and I therefore suspect vacanapatha is used throughout for simplicity, such uniformity being one of the characteristics of oral literature.
It seems clear to me, then, that in the EBTs this word is restricted to unpleasant kinds of speech, presumably rude and offensive language.
The commentary to the _garudhamma_s, however, seems to narrow down vacanapatha to “correcting”:
Vacanayeva vacanapatho. … Tasmā bhikkhuniyā ādhipaccaṭṭhāne jeṭṭhakaṭṭhāne ṭhatvā ‘‘evaṃ abhikkama, evaṃ paṭikkama, evaṃ nivāsehi, evaṃ pārupāhī’’ti kenaci pariyāyena neva bhikkhu ovaditabbo, na anusāsitabbo. Dosaṃ pana disvā ‘‘pubbe mahātherā na evaṃ abhikkamanti, na paṭikkamanti, na nivāsenti, na pārupanti, īdisaṃ kāsāvampi na dhārenti, na evaṃ akkhīni añjentī’’tiādinā nayena vijjamānadosaṃ dassetuṃ vaṭṭati.
Vacanapatho just means speaking to. … Therefore a bhikkhunī should under no circumstances take a position of authority or of an elder and instruct: “Go forward like this and return like this; dress like this and put on your upper robe like this.” If she sees a fault, however, she is allowed to show an existing fault in this way: “Previously the great elder did not go forward or return like this; he did not dress or put on his upper robe like this; he did wear such a robe or use make-up for the eyes.”
Although the commentary uses the words ovādati and anusāsati, which often have the wider meaning of “teach,” the above passage makes it clear that only direct criticism is proscribed. In any case, even this interpretation from the commentary seems to be the outcome of a developed understanding of vacanapatha not found in the earliest texts.
There is nothing in the above, either from the Pali EBTs or the commentary, to stop a bhikkhunīs from teaching monks. And so I remain unclear how this custom has taken hold in Sri Lanka.