Let's say that it was a phenomenon that lasted, perhaps, a few days. There may have been no awareness of what had happened to all the disappearing ships, no way of connecting it to the use of dilithium at warp.Saru makes up that Discovery wasn’t at warp when the Burn happened and that’s why it didn’t explode, but from what they said earlier I think the excuse doesn’t really hold: even ships not at warp seem to have exploded, as long as their warp cores were powered up.