If I were in college and submitted an essay to my professor, he doesn't need to ask if it was my work; it's implied. If someone posts a body of text on a forum without citing the source or being clear about authorship, it is similarly implied that the body of text is the submitter's own work. That is plagiarism. If the plagiarizer is questioned about the authorship and insists that the work is his own, then it is lying as well.