What is the meaning of "This is what you used to (bihi taddaʿūn)"?

Imām Ibn al-Jawzī (d. 597/1201) summarized the scholars' opinions in his book "Zād al-Masīr" as follows:

1. The Arabic verb form taddaʿūn (with doubled letter, form VIII) carries the same meaning as tadʿūn (form I): “you call/invoke.” It is derived from duʿāʾ. [Al-Farrāʾ, Ibn Qutaybah] The reading of Yaʿqūb and others is based on form I.
2. The meaning is: “This is the thing (i.e. the outcome) for the sake of which you used to make false claims, i.e. that you would never be raised after death."" [Al-Zajjāj]"