Yes, but it is not correct to claim Macbook Pros are Iris Pros. They are Intel graphics.

Intel GPUs (including the UHD Graphics 630) can be used with external monitors. HP, Dell, Lenovo, Acer etc all do it.

In fact, in 2013-2015, Macbook Pros were available with ONLY Intel graphics, and supported external monitors.

The reason why you cannot do it with dual-GPU Macbook Pros is that Apple has their own display muxing solution that requires the discrete GPU to be powered up to use external monitors (presumably the ports are attached to the discrete GPU).

Of course it is. The 13" MBPro and Intel Air work great with external monitor.