I can confirm that. We had a single-AZ RDS instance whose underlying storage (a "magnetic" EBS volume) unrecoverably failed, according to AWS. It had to be restored from a backup. Fortunately, "point-in-time recovery" meant there was very little data loss, just some downtime.

(Not that data loss or downtime mattered for this instance, which was just used for internal testing.)

Power-up is very stressful for hard drives, so it's not too surprising that some failed when the power turned back on. EBS does offer spinning rust storage options, so maybe mLab was using those for some of those failed volumes. I don't know if the same is true for SSDs or not.