You can put hashtags on your Tumblr posts. And people sometimes use it as a post scriptum

When you make or reblog a post, you can add tags at the bottom. Ostensibly these are for searching/categorization, but people often use them to write out responses to posts so that their followers can reblog the it without bringing their comment along (Tumblr just puts all replies into a single extended post so it's a bit cumbersome to have long comment chains). The tags are visible in the "notes" section of the post, so people can still see it.

When you see a screenshot like this, it likely means that the response was made by someone else and the OP self reblogged it because they thought it was important.