AS most likely does not allow mixed cabin partner bookings on DL metal. Just because DL allows it with Skymiles does not mean AS will allow it when using AS miles. What was the mileage amount on DL? DL charges 70K for one-way low-level J.
That would be news to me.
And since the AS website is offering the mixed reward, you would think the agents would be able to see it. Or is the website just that broken?