Insertion is prohibited for anyone after the Companions; an explanation of an unfamiliar word is, however, permitted.
In the example, يَتَحَنَّثُ is explained by وَهُوَ التَّعَبُّدُ.
Keep the distinction: explaining a word and taking one’s own words to be Prophetic speech are not the same. Permission to explain does not authorize every kind of addition.
1. Unfamiliar word: a word whose meaning is not clear to the listener.