Note that that book describes the AT&T Unix system internals as they were 30 years ago. You can't assume things are done the same in modern Unix and unix-like systems.
In any case regardless of how pipes are implemented internally, while for regular files or other seekable files, the byte offset is something that belongs to the open file description (I suppose that's what your book calls a file table entry). That is, two processes opening the same file independently will have each their own offset within the file. One process reading from the file doesn't affect the offset of the other process.
For pipes, all file descriptors of all processes open on a pipe share the same offset. Or in other words the offset belongs to the pipe. So it makes sense to store it in the inode rather than duplicating it in all the open file descriptions.