What is an emulator POD?
I have witnessed this type of hardware emulator used about 20 years ago at HP Personal Software Division... in the days of single core 80286... The target system's CPU chip (such as intel x86) is replaced by the hardware emulator pod. Physically the hardware emulator consists of a box (maybe the size of a big laptop computer) and a pod, which is a smaller connector that fits into the CPU socket. In use, the CPU is physically removed from its socket on the motherboard, and replaced by the hardware emulator's pod. Inside the hardware emulator's main cabinet is an FPGA or a specialized ASIC that is equivalent to the target CPU, but with some additional debug capabilities. Then that is in turn connected to a second computer, which is used as the debug console.

This combination of a small connector pod together with a larger cabinet, gives a firmware developer direct access to the internal workings of the target CPU, running at full speed. Mostly we try to avoid this due to the very high cost and awkward interface of such a system. But if you're debugging a system and need to set a breakpoint on a kernel interrupt, and there simply are no debug tools at that level, then a hardware emulator may be the only way out.

What Jack Ganssle is referring to here is that the emulator's pod is physically larger than the target CPU which it replaces, and may have some additional mechanical requirements that the motherboard designer should take into account. Specifically, the target CPU does not have an umbilical cord linking it to a whole other cabinet of electronics. This consideration is applicable if you were designing an 80286/80386/80486 motherboard 20 years ago, and have a hardware emulator in mind.

Not every CPU has or requires a hardware emulator. Many newer CPUs are designed with hardware debug facilities on-chip. For today I'd interpret Jack Ganssle's advise as meaning you should consider what hardware debug tools are available for your target CPU, and ensure that your PCB design can support those tools. If you look for example at STMicro's STM32F "Nucleo-F401" evaluation boards, they have a software debug header that supports a debug protocol specific to that model CPU. When you design your own STM32 based board, it's smart to include that debug header as part of your design. Even if you leave the debug header unpopulated, even if you never end up needing it. Having the debug header available, someplace where you can reach it, gives you the capability to examine and debug the firmware directly on the target board.