This is true, but as it happens, the general conclusion (per the OED) is that syllabus isn't an authentic Latin word at all, it's a corruption or misreading of Greek sittybos as sillybus (spelling varies). So even though it's modelled after the Latin, it's already plural if the surmise is correct.

Now this is top tier pedantry, ya love to see it.

Edit: if you would've subtly but deliberately screwed up that idiom, this would be elevated to a true masterpiece