Khandker is the chairman of the forum comprising sector commanders of 1971 war who are still alive.
In a joint statement, the organisation on Saturday said they were "baffled" at the "baseless" information presented in the book on the historical Mar 7 speech.
"This is utterly unfortunate and devoid of subject-matter," it said.
Khandker was the planning minister during the Awami League's last term in office.

In his book, '1971: Bhitore Baire' (1971: Inside out) published on Wednesday, the deputy chief of the Mukti Bahini claimed Bangabandhu Sheikh Mujibur Rahman had said 'Joy Pakistan' at the end of his Mar 7 speech.
Parliament members on Thursday flayed Khandker and demanded the book be outlawed, but Finance Minister AMA Muhith on Friday said no book should be banned.
In the statement, the forum's senior vice chairman Maj Gen (retd) KM Shafiullah and 20 other members said the book contained Khandker's own thoughts which were in contradiction to the organisation's policy, ideal and historical truth.
"The statement by the Liberation War's deputy chief after 42 years of independence is deceptive."