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.
If you want to jump to the conversation/replies to the OP's multi-post, this is a lot less scrolling than if each part of the OP's multi-post was automatically expanded. There's an advantage of the setup for you. This might be desirable as it could encourage participating in the conversation, though an argument can be made that it comes with a risk of people skipping the content and joining the conversation with incomplete context.
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.
