Quoting the original blog post:

The backup file contains the textual content and meta-links to assets (images, videos, any files that you’ve sent and received) on Wire servers, not the files themselves. This helps to keep the backup file sizes small. When you import history backup into a new device the meta-links mean that all your images and files will show up in the right places in conversations.

If you create a backup file, then delete some chats and reimport backup then the chats you deleted will be back.