A tape contains files but not a filesystem*. Using the word 'file' for both is a bit misleading. A file on a tape is nothing more than a stream of bytes closed with an EOF mark. That is all. As wurtel wrote, a "file" on a tape has no name, no attributes. In other words, there is no catalogue of these files. Catalogue = filesystem.
cp deals with files in file systems. It copies the file named on the command line from one filesystem to a different directory on the same or another filesystem. Think about it: the fact that there is no filesystem on a tape means that there are no file names and there are no directories. cp has nothing to do with tapes.
It is not by accident that the book author recommends tar to store several files on a tape. TAR means Tape ARchive. The TAR format and utility was created to be able to package the content, name and attributes of several files along with their placement in a directory tree and write the whole thing onto a tape. Not always, but frequently this is how you want to store files on a tape.
*Footnote: there IS a tape file system, which is called LTFS. This is a relatively new thing. The tape is divided into two partitions, the first contains the catalogue, the second contains the file contents. LTFS behaves like any usual file system, you can copy with cp, or even drag-drop files. But if you simply mentions "tape", I assume you are talking about the usual, basic tape configuration, without LTFS.