my best guess, is that when booting from a usb drive (either always, or when the usb descriptor says its power hungry), the firmware will check if a usb-c PD PSU is present and if it hasnt confirmed it can get the expected 5A, it will display that error message, and then give a 1 long 1 short blink code
you can then click the power button to silence the warning and continue booting the warning also says that usb_max_current_enable=1 would disable that safety, but the link in the warning doesnt go anywhere
The official Pi5 PSU is PD compliant. The Pi5 exchanges signals with the PSU. If it gets the correct response back, it "knows" that it can get 5A and sets the USB (output) current limit to 1.6A. Otherwise, it sets that USB current to 0.6A. If the PSU doesn't do the PD handshaking, the Pi5 won't set the higher USB current limit.
For anyone looking to do something similar, there are some other to consider. Some power supplies have low reserves of power to supply peaks. If everything switches on at once, the voltage may dip and the Pi is the most sensitive device. Your supply appears to have way more than you need but the description says it is designed for LED lights which are not sensitive.
USB boot is disabled by default when connected to a 3A power supply.
Set usb_max_current_enable=1
in /boot/firmware/config.txt
to enable USB boot.
Set usb_max_current_enable=1 in /boot/firmware/config.txt:
$ sudo nano /boot/firmware/config.txt
Find [all]:
[all]
usb_max_current_enable=1
Save and Reboot
Alternatively, you can press the power button a single time on a failed USB boot to temporarily enable "usb_max_current_enable" and continue booting. However, this setting will not persist after a reboot if enabled by pressing the power button!