Yes, a snapshot actually sets the original disk to read-only
This portion of your statement is not true. The file attributes are not changed and the parent disk remains writable and there are many cases where users have gone and modified a parent disk when snapshots existed and then get the error that the parent disk has been modified when trying to run the VM again.
and writes all changes to a new virtual disk (vmdk file).
This portion is true however the the Read Only Attribute is not set on the Parent Disk just because one takes a Snapshot.
This means reverting to a specific snapshot will discard all changes that were made after it was created.
This also is true however the reason I've pointed out that as written some of what you said is factually inaccurate, (aside from the fact it is,) and understanding you're just trying to explain in simple terms the overall context of the process nonetheless it's dangerous to assume that the Parent Disk is safe from being modified simply because a Snapshot exists. If one opens that Parent Disk by other means while it has Snapshots, as an example mounting it on the host as a local volume while the VM is not running, it will break the VM when attempting to run normally again. So my intent here is to make sure users understand that the Parent Disk's Attributes are not set to Read Only and can be modified by other legitimate uses and means that can break a working VM if one accidentally or intentionally access the Parent Disk by other normal means while the VM is not running.
