I see button options to sync and reload, are these calling the callback methods? preSyncCallback and postSyncCallback do not exist when using listAttributes() on the HDA node so i'm inclined to believe the docs are just outdated.
If you're looking at the Sync and Reload buttons in the Asset Options tab of the Attribute Editor, you should see the AssetCallbacks tab right below. There should be string fields to enter the names of the mel procs to use as callbacks.
Which version of Houdini do you have? Those attributes were added for 17, it would be a shame if something happened to them.
What happens if you do: attributeExists “preSyncCallback” “yourAssetNode”