Nowadays it is common to salt and encrypt passwords when you save them to a database. The software can reverse this process and the passwords are stored save even if the database is not encrypted.
But it is a performance-based operation, so especially for big platforms somewhat cpu intensive.
A Meta spokesperson said the company took immediate action to fix the error after identifying it during a security review in 2019, and that there is no evidence the passwords were abused or accessed improperly.
Sure, you can guarantee this, as all the other corporations did in the past.