I don't get that. We want to collapse the post preview so we need to collapse a post if it's marked as a preview. What you said is the exact opposite, isn't it? 

If you want to imply that we need to check if the post is a preview before collapsing it then I don't agree. <code>modifyPostPreview</code> uses the variable <code>post</post> which is modified to look like a preview. Then I try to collapse just that post (I look for the expander in that post) so it is definitely the post I want to collapse.