Stock image dump for running Magisk necessary?
No. Far more easier is to use the flashable ZIP that does all- That performs all the steps: backing up the boot image, creating a patched version, flashing that, installing Magisk Manager (thanks to Izzy in comments)
But to clarify on the instructions
What’s the rationale for having the stock boot image dump beforehand?
For devices that don't have custom recovery but you would want to root
Magisk needs the stock boot image for two things
To be able to patch it so that Magisk can be installed.
If you wish to uninstall Magisk, it replaces the patched boot image with stock boot image
Furthermore, it is unclear to me how to create that dump
For MiA1 you can see the details on this XDA thread. In essence:
You need to have fastboot image of the stock ROM (for MiA1 they are released every month)
Extract boot.img and store it on your device
Install Magisk Manager .Tap on Install on the prompt and then "Patch Boot Image File". Select the untouched boot.img through your File Explorer. Wait for it to do its thing.
The patched_boot.img was created and it should be located at /storage/emulated/0/MagiskManager/patched_boot.img.
Copy the patched boot.img to your PC and fastboot into it (fastboot boot patched_boot.img)
From Magisk manager Direct Install and you are done
If that is too much work, you can wait for somebody at XDA to provide patched boot images
Edit : See this for more How do you root a device with Magisk when it doesn't have a custom recovery