How to decrease size of a root volume EC2 Amazon Linux2?

Reducing the size of a volume is difficult (something that is inferred from the complexity of the instructions in the other post). That's because the operating system has already allocated the blocks on the disk to whatever file/block index mechanism is being used - so this isn't restricted just to Amazon Linux 2.

In short, to reduce the size of a volume you must create a new volume; copy all of the files across to it; and then (in the case of the boot volume) ensure that whatever boot mechanism is required by the operating system is present.

I think it would be a whole lot easier (and less risky) to create a new instance with a smaller boot volume then copy the files across.