Actually, it is. Any Swahili resource you look at will list kunywa as a monosyllabic verb.
There are two bi-syllabic words that are sometimes treated as monosyllabics, kwisha/kuisha and kwenda/kuenda (which is why some sentences in the course use -enda and some -kwenda, and unhelpfully I do not think there is any rule to tell us which one to type. It could just be down to which team member wrote that particular sentence).
But kunywa is considered a proper monosyllabic verb.