The goal was to show how little you have to do to do a fairly feature rich carrier board for the Compute Module 4. Also to demonstrate that you don't have to be scared by high-speed signals like HDMI and that they can still work fine in very bad conditions like a single layer PCB like this one without a reference plane underneath.
Please only use this as a starting point if at all and take a look at the open reference design by the Pi foundation for a more in-depth carrier design. This design skips several saftey features like load switches on the USB and HDMI, most of the time that's fine but you should make sure that you really don't care about that.
Also do please attempt to do some impedance matching. An online calculator to get the correct trace width is already an ok approximation for most use cases.
4 Layer PCBs are to be had for 5$ and less now a days.