Why does the image of the Expansion Slot Utility in this article show that Pool B is 88% allocated when one x4 device is present in Pool B and zero are present in Pool A ?

the answer is in the next paragraph from the one you quoted.

PCIe bandwidth

The M2 Ultra chip provides 32 lanes of PCIe gen 4 to the system, with 8 lanes dedicated to the internal SSD. The M2 Ultra chip connects to the PCIe slots through a PCIe switch and provides 24 lanes of gen 4 bandwidth.

Pool A provides a maximum of 16 lanes of gen 4 bandwidth and Pool B provides a maximum of 8 lanes of gen 4 bandwidth.

These built-in components are connected to the system through the PCIe switch and assigned to Pool B:

• SATA controller
• 10Gb Ethernet controllers 
• Wireless (Wi-Fi, Bluetooth) controller
• USB-A port
• PCIe Slot 7 (Apple I/O card)

This means that a system will always show a percentage allocated to Pool B, even if no PCIe cards are installed.

Yes, over-subscription is allowed, and you have to deal with any slowdowns that occur.

However, one of the highest users of I/O bandwidth, disk drives, tends to be very intermittent, and generally one direction (either inbound or outbound, but not both at once) at a time. So in general, you will not notice a modest over-subscription, because actual slowdowns will be rare and brief.

It would still be wise to be thoughtful about what gets assigned where.