at 21:10 Tone claims that inappropriate posts can be taken down and Jeff correctly states that everything recorded in the blockchain will stay there, even if the creator of the post him self chooses to edit or delete content.
For those interested in the details of that, I did a practical experiment on exactly that a few days back.
As a person not associated with the steem creators I was able to recover deleted/edited posts directly from my own node independant of steemit.com.
https://steemit.com/steem/@m0se/steem-is-immutable-think-twice-before-you-post-you-can-t-delete-later-on
The article provides a abstract description on how editing works in a blockchain environment and a practical example with instructions how to recover original posts