That is correct. If your printer control board doesn't have a bootloader and you try uploading over the USB cable, you will have an error during the actual upload process. But the code must successfully compile before the upload process even starts, so the current compilation error you are getting is completely independent from the presence or absence of a bootloader.
The part you need to focus on is step 5 of the first video, where you add the A8-specific Configuration.h and Configuration_adv.h files to Marlin:
https://youtu.be/ePgpzkjriso?t=470
Make sure you have copied the right files to the right place.
Try this:
- Open Marlin in the Arduino IDE.
- Click the "Configuration.h" tab.
At line 24, you should see this:
#define CONFIG_EXAMPLES_DIR "Anet/A8"
If not, it indicates that something went wrong while you were following step 5 of the video.