I agree with Jo - technical glitch! Most likely the person hit the button for "Reply" which automatically quotes the entire post in a new reply - but got distracted and didn't get around to writing anything. Or they were using their phone, and their finger inadvertently hit that button, and they didn't know that they have a few moments to delete the post.
Many people use the Reply button, but I find quoting the entire post to be unnecessary, and a waste of screen space. I prefer to just mention the poster by name if I want to respond. I do, however, like to quote specific sentences. To do that, you just point and drag to highlight the text you want to quote, then click on the "Reply" flag that appears on the tail end of the highlighted text.