The midjourney founder talked about it in his "office hours" in the weeks prior to inpainting launch, they were ready with inpainting before discord was done with UI support for it.

The way inpainting in midjourney works is you click "remix region" on the message where the bot posted the generation and UI pops up that lets you select regions of the images, with two different types of shape selector, a rectangle selection tool and a curve selection tool. You send your message back to the bot, which includes those regions, and it only redraws those regions.