The law does not actually say that it's up to the IG to determine whether it's urgent or not. The term "urgent" is defined in the text. Thus, whether the IG says it's urgent or not, there is an independent standard for urgency.
There is not a single statement in that law that says if the IG says it's urgent, then it's urgent.[1]
Those in the administration are bound to follow the OLC's judgment in interpreting the laws that apply to their official work. Obviously, in this case where the AG is mentioned in the complaint, this is not ideal, but Maguire went by the book. When he had questions about how to react to the complaint and the IG's report in this (as he said) unprecedented case, he went to the highest authority in the administration for clarification.
I am not commenting on the OLC's response to Maguire's query, only the appropriateness of turning to the OLC.
[1] I had the same opinion as you expressed. I thought that if the IG says it's urgent, then it's urgent. My wife, who is an attorney, went to the text of the law and pointed out that's just not what it says.