Involuntary support for legacy kernels?

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I managed to boot the Raspberry Pi official kernel with v1.35 firmware and have complete access to all of the Raspberry Pi's peripherals! However, I did not recompile the kernel with UEFI support and am confused how this happened. Here's how I did it (these steps were performed on a USB stick):

1. Download the Raspberry Pi kernel and save it as /boot/kernel8.img.efi. Make sure it ends with .efi.
2. Change the boot order USB -> SD Card -> Network Boot with raspi-config.
3. Insert an SD card (with a valid Raspberry Pi OS image) into the SD card slot.
4. Reboot.
5. Upon seeing the Raspberry Pi logo, press F1 to enter the UEFI shell.
6. Type fs0: to switch to the boot partition of the USB stick.
7. Type ls -b and make sure that kernel8.img.efi exists. If not, try fs1:, fs2: etc and find it.
8. Once you find kernel8.img.efi, type kernel8.img root=/dev/sda2 rootwait ro. This is NOT a typo, do not include the .efi.

Somehow, this only works for me when there is an SD card in the SD card slot. I have no idea whatsoever how this works at all. This opens up a new opportunity for a "legacy boot" mode for kernels that don't support UEFI. This also makes full Debian support a real possibility.