Q: Noatime flag

as is says in the article:  "DISCLAIMER: Applying any of these tweaks is at your own risk"

Apple and the Apple Support Community does not recommend "tweaking".

"MacOS (like other unix-based systems) by default records last access time for every file. I.e. every time you read a file, a write is made on the filesystem to record this action." : Leave it as is, or you might experience problems in the future.

Try

sudo mount -vuwo noatime /

mount | grep noatime

from an Applications -> Utilties -> Terminal session

You will be promoted for your admin password.  Type the password and hit <return>.  You will NOT SEE ANYTHING while you type the password, until you hit <return>

Does it work.

if so, it can be done, at least manually.

Then review your .plist carefully to see if there is any deviation from the example.

Also look in Applications -> Utilities -> Console for any error messages that may related to running your .plist

Have you actually run that command to see if it will remount with noatime…

sudo mount -vuwo noatime /

Then check the mount info…

mount

Also RTM

man mount

I really do not understand why you need a launchd job to do the task that /etc/fstab does (maybe I am too old & out of date!)

man fstab

Create a fstab file (not fstab.hd) if you plan on using that, the manuals examples use the UUID which is a good idea, get that from the info section in Disk Utility.

Otherwise check your launchd job is actually loading.

sudo launchctl list

The second column should be the exit status (assuming it ran) - you have rebooted or loaded the job manually too?