There were plenty of straight flats with enough capacity available at the time to carry these tanks. I'm not sure if the Magors had arrived yet, but the 39095 series appears to me to have been available. And they had a capacity of 100 tons.
I'll also note that the field manual for this tank shows the loading only for the Magor flats. NOT for depressed flats that might have been in foreign service. That certainly doesn't mean that info wasn't elsewhere, but it wasn't "important" enough to have been included in the manual.
If the photo was taken in the US, I s'pose the setup could have happened because of an unusual shipment through territory with low clearance.
I'm a just finding it interesting that there appears to be only ONE photo of this car or any of its brothers. And no record of the series available.