I think this is a totally baseless comment.
1. Have you worked at Facebook as an engineer before? Do you first hand know that most engineers working on FB iOS app lack the experience and interest?
2. Have you reverse engineered the FB app and verified that the app lacks signs of proper system or application development?
There could be a good enough reason as to why the app re-wrote core iOS libraries - most popularly performance and missing features. Also, tens of thousands of classes don't directly translate to increased binary size - its usually the dependencies of the code that results in such large binary sizes.
Overall, if Facebook wanted to optimize its app size to be less than 491MB, they would have.
Maybe its on their roadmap, maybe they decided most of their userbase have the ability to download 491MB and it isn't impacting the growth funnel, heck maybe some of the engineers actually wanted to work on optimizing the app size but it was shot down by management. Slamming engineers who work at Facebook and calling them inexperienced is just mean.