No it couldn't.
On models with 2 graphics card, only the discrete card is connected to the external ports, the Iris isn't. It has nothing to do with OS X and 100% with hardware.
AutoCad is a very light use on a PC despite what they would have you believe. It is essentially the same software from the mid 90s, with various marketing spin and annual additions.
There should be NO issue whatsoever using any Macbook Pro for Autocad, I use AutoCad 2017 on a 2011 Macbook Air, 4gb ram, Intel HD3000 graphics on an external 1080p Monitor with large files, and it runs very well indeed with little to no thermal throttling or fan noise.
An Iris Pro 5200 graphics as similar in power to a Nvidia 730m, i.e. about 2/3 the power of a Nvidia 750m, so it is plenty powerful enough for most applications that most people use.
I have used AutoCad for exactly 30yrs. It is written to use the CPU and not the GPU for graphics.
While there is a switch in auto cad to use dedicated graphics, it goes against the grain of how the software works and is of no value at all except in very extreme cases.
It sound like a reinstall of the software is required.
my two cents.