It's true once data is into a blockchain, it's near impossible to change it. Edits are done in Steem by adding new posts or comments to the blockchain that use the same permalink title as the previous post. The edits contain the changes and some special characters that tell the Steem front end how the post is edited by the new edit transaction.
I edited this comment after posting the above paragraph to illustrate. I will edit it again to show you what the edit transaction looks like.