If it is relevant at all, the threshold of originality applies to the allegedly-copyrighted source consumed by Copilot (as regards bare infringement, not wilfull infringement). If that doesn't meet the threshold, it is not copyrightable. If it is, unauthorized copying not within a copyright exception (e.g., fair use) is infringement.

I can't see any case where originality of the snippet presented by Copilit matters (if Copilot were a person, it would matter to determining if the snippet on its own was copyrightable by Copilot, but still wouldn't be relevant to whether to original copyright was violated.)