The pony bottle is listed as ’recommended’ equipment, not mandatory:
Pony bottle — Although the pony bottle is not generally considered
mandatory for all penetration dives, its use is highly
recommended in addition to an alternate-air-source second
stage or inflator regulator. A pony bottle is a totally independent
air source and serves to provide an extra margin of
safety in an overhead environment. It is easier for you to exit
the wreck using your own pony bottle than sharing another
diver’s air. The deeper you dive, the more useful it may be.
On some wrecks, local divers may consider the pony bottle (or
other redundant, independent air source) normal equipment.