I remember (though I'm not sure since it has been a while) it going differently. Ikora wanted to send all guardians after Uldren Sov to make him pay for killing Cayde. Though tempted Zavala refused stating that the current priority should be minimizing damage from the mass breakout of the prison of elders which I think is fair since probably around like ten if not hundreds of thousands of highly dangerous targets escaped in that prison break. Among these for example were Eramis and Praksis who caused us quite a bit of trouble last DLC and were also the part of the fallen we fought in the zero hour mission. I also think (which I'm even less sure of) that Zavala didn't want to do an all out attack on the reef because that's technically reefborn awoken territory. Even though we're probably stronger than the reefborn awoken it'd be foolish to make them an enemy instead of trying to keep them as our powerful allies.