Judging from the context Ohrmazd knows his creatures can be corrupted and that in this way he tricks the evil spirit into accepting a proposed time for their final fight (making sure they are not fighting constantly). Ohrmazd, using his omniscience knew the end result would result in the evil spirit being disabled.
And the evil spirit shouted thus: 'I will not depart, I will not provide assistance for thy creatures, I will not offer praise among thy creatures, and I am not of the same opinion with thee as to good things. I will destroy thy creatures for ever and everlasting; moreover, I will force all thy creatures into disaffection to thee and affection for myself.' And the explanation thereof is this, that the evil spirit reflected in this manner, that Ohrmazd was helpless as regarded him, therefore He proffers peace; and he did not agree, but bore on even into conflict with Him.
And Ohrmazd spoke thus: 'You are not omniscient and almighty, O evil spirit! so that it is not possible for thee to destroy me, and it is not possible for thee to force my creatures so that they will not return to my possession.'
Then the evil spirit, unobservant and through ignorance, was content
with that agreement; just like two men quarreling together, who
propose a time thus: Let us appoint such-and-such a day for a fight.
Ohrmazd also knew this, through omniscience, that within these nine
thousand years, for three thousand years everything proceeds by the
will of Ohrmazd, three thousand years there is an intermingling of the
wills of Ohrmazd and Ahriman, and the last three thousand years the
evil spirit is disabled, and they keep the adversary away from the
creatures.
It might even be because Ohrmazd is convinced that by then he will have made sure Ahriman is not feared anymore by his creations.
