Can Dragonbait read or write Common, since he is able to understand it?

In D&D 5e, Tomb of Annihilation includes Dragonbait, a saurial. His stat block claims that he can understand Common but not speak it, due to the strange way in which saurials communicate. From Tomb of Annihilation, p. 218:

Languages understands Common but can’t speak

Is there any evidence, either in 5e (which I assume is just what’s presented in Tomb of Annihilation) or anything from previous editions of D&D, that suggests that Dragonbait (whether it’s about Dragonbait specifically or saurials generally) can read or write Common, since he can apparently understand it?