From the manual's Macro Syncing page:
WARNING: All of the existing macros on this second Mac will be destroyed when you click the Open Existing button. If you have any macros on the second Mac that you wish to preserve, export them first, and then after syncing is enabled, import them (and they will then be synced to your other Macs).
Might it be a good idea if pressing "Open Existing" had a default option to trigger an export?
✔️ Export all macros on this Mac before syncing
That might arguably take some responsibility off the user, but it might help a confused beginner, and it seems ironic for an automation application, of all things, to suggest an extra manual step here. 