Persons specified in section 23 are excluded from registration. If such persons are receiving inward supplies liable to tax on reverse charge basis under section 9(3), they will be liable to register by section 24 and comply with other requirements of law. But, if such persons do not attract section 9(3), then they can remain unregistered and therefore not attract section 9(4) too.
(ICAI FAQ PUBLICATION 06-09-2017 REVERSE CHARGE: FAQ NO.40)