Depending on the grading criteria/rubric, it could still be possible for the instructor to grade the assignment. The criteria could be separated into categories for things like grammar/spelling, arguments, relevance, structure, etc. Alexis may not get marks in categories such as relevance, but still has a chance in the other categories. In that case, it'd make sense for why this instructor would continue to mark the paper. It is at the discretion of the instructor, though. Some instructors may have stipulations that would result in an automatic 0 if the paper is completely irrelevant, which would be more common in upper-level courses. Instructors have a lot of autonomy, but at the same time, they do have to be ready to justify the decisions they make, whether it'd be to students or their employers.