The method to install to eMMC’s have changed, in an effort to not have to maintain 2 separate images for each supported device and edition.
Read the release post for 20.08 again:
New application for flashing images
We have been working on a new GUI app to make it easy for users to flash a Manjaro ARM image on to an SD card or eMMC card.
Simply install manjaro-arm-flasher and launch the app “Manjaro ARM Flasher” from your menu. This will download and flash the image of your choice.
You can also manually run the application (if you are not running Manjaro ARM) by downloading this python script , make it executable with chmod +x manjaro-arm-flasher and then run it with sudo ./manjaro-arm-flasher . It requires python-blkinfo 0.1.3 to be installed, which can be installed with pip install blkinfo .
This software is still in early development and does not include all the features we want yet.