Posts aren't limited to an arbitrary length in activity pub, this is the same UI logic for showing a thread in the main feed. Twitter does something similar for bluecheck long posts.
The fade effect on the line of text shown is in line with how "click to show more" is done in many places all over the web. It took me a thoroughly minimal amount of mental capacity to realize there was more. I clicked, and got more. It's really not that confusing.