The Indemnity Ordinance was expressly made by the President in exercise of the powers conferred by clause (1) of Article 93 of the Constitution. So, there was no extra magic in the Indemnity Ordinance which in normal times was required to be laid before the Parliament for its survival.
To give approval to a law is a qualitatively different act from enacting the law itself. An Ordinance when approved in the Constitution remains an Ordinance and it does not become a part of the Constitution. [Per A.T.M. Afzal, C.J] Shahriar Rashid Khan Vs Bangladesh and others, 18 BLD (AD) 155.

Para 3A and 18 of the Fourth Schedule
The Indemnity Ordinance was promulgated to restrict the taking of any legal or other proceedings in respect of certain “acts and things done” prior to the promulgation of the Ordinance. The protection which was been given under paragraphs 3A and 18 to the “acts and things done” relate to those that were done in exercise of the powers derived or purported to have been derived from the Martial Law Proclamation. They have been ratified and confirmed and are declared to have been validly made, done or taken and shall not be called in question before any Court, Tribunal or authority on any ground whatsoever. Evidently, the acts and things which were done on the 15th August, ‘1975 to which indemnity was sought to be given by the Indemnity Ordinance were not done in exercise of the powers derived from the said Ordinance. Therefore, they have no nexus with the “acts and things done” referred to in paragraphs 3A and 18. [Per A.T.M. Afzal, C.J.] Shahriar Rashid Khan Vs Bangladesh and others, 18 BLD (AD) 155.

Indemnity Ordinance, 1975 (Ordinance No. L of 1975)
Indemnity (Repeal) Act, 1996 (Act XXI of 1996)
Indemnity Ordinance, 1975, (Ordinance No. L of 1975) having been repealed by the Indemnity (Repeal) Act, 1996, (Act XXI of 1996), this repeal can not be legally construed and meant to have negated the nature of those "admitted certain acts and things" done prior to the promulgation of the said Ordinance, it was neither the intention of the legislature to negate the nature of the offences committed by the persons involved in the incident on 15 August, 1975, and thus the prosecution erred in law in initiating a criminal case against the petitioners for the charge of murder and this fact having not been addressed by this Court in its judgment is an error apparent on the face of the record. ...Major Md. Bazlul Huda (Artillery) =VS= The State, (Criminal), 2021(1) [10 LM (AD) 581] ....View Full Judgment