Looking now on EF, I'm seeing O1 X0 on that flight. Looking on delta.com, that business class seat is offered to me for 65,000 miles and 96.48 USD.
Maybe you got a subpar agent. Maybe s/he didn't understand you wanted business class (assuming you do). Or maybe the availability EF alerted to you was taken and then re-released, or taken and later replaced with new award inventory.
Either way: if you want that seat and you're online reading this: try again!
I did indeed get a less than competent agent last night. I was very specific about the class of service (Business). This morning I called again and was lucky to get a superb agent who found the inventory and was done within five minutes. I was looking for SYD-CAN-DEL in Business. It was 80,000 Skymiles and USD 132.
So, from my first experience using Expert Flyer, I can say with some certainty that EF inventory is quite reliable, although there is at least one post in the
"Using Skymiles on SkyTeam and Other Partners" thread over on the Delta forum that suggests that DL agents can't see CZ inventory that shows up on Expert Flyer.