Because a permalink with just an ID and no slug text is still a valid link for that post on Tumblr. Try it.
Some people link to those like that.

The additional "index.html" in the directory above catches any search engines at least going to just http://blog/post/postid/ without a slug, and lets them know the actual target for merging. Otherwise a 404 will return, which could risk the directory being delisted by a search engine due to confusion. (in theory, a root shouldn't return a 404 and then have a page in it, although plenty of people make this mistake I think it's best not to raise any "Red flags" as Google approaches)

Also, it's possible to link to any "slug" text when linking to a Tumblr. I can make up my own for any like,
http://blog/post/postid/some-stupid-title .... or http://blog/post/postid/some-even-more-stupid-title

.... And both will work as permalinks. This can also create some confusion as truncated links will still work. Unfortunately, without real redirect rules, it's impossible to catch these without manually creating them ... but I think the code above makes sense because the permalink with just an ID and no slug is still valid on the old platform. Can't hurt in my opinion.