I don't know about the latest Intel integrated GPUs, but for the original ones it was not a unified memory arrangement. Rather the integrated GPU used a dedicated chunk of main memory. It was the same banks of memory, but was treated as separate. If you wanted a texture to appear on-screen, then it had to be moved from the normal pool of memory to the one reserved for the GPU.

This reserved pool of memory was even hard-set at boot time, and usually the size of it was determined based on the amount of total memory (for example at one point MacBooks with 4GiB of memory allocated 1GiB for memory, ones with 8GiB allocated 2GiB).

What it sounds like Apple has done here is heavily blur those lines. One example would be to load a texture into a memory location, make a change with the CPU, then let the GPU do some modifications, check something with the CPU, then apply it to some shape and display it. On current systems that would mean multiple copy operations of that texture back and forth between segments of memory (across the PCI bus on a dedicated card). Here that might all be done with no copies at all (just handoffs).

iGPU doesn't mean unified memory.