the example was generated on a mac, which uses the FreeBSD userland utilities. seq is not included in non-GNU user-land utilities (as it is not POSIX), jot is the (more-or-less) equivalent of seq for BSD systems.
$ uname -s
Darwin
$ type seq
seq is /usr/bin/seq
The seq command first appeared in Plan 9 from Bell Labs. A seq command appeared in NetBSD 3.0, and ported to FreeBSD 9.0. This command was based on the command of the same name in Plan 9 from Bell Labs and the GNU core utilities. The GNU seq command first appeared in the 1.13 shell utilities release.