The settings page doc doesn't give any additional details regarding the option_name argument. Either you create it or it will use the ID. I am not sure if this affects the ability to save meta field data for a settings page into a custom table. The field group itself allows defining the storage type for a custom table.
I noticed upon trying to save it creates an empty serialized array with the option_name in wp_options instead of saving to the custom table. I wondered if perhaps it was because I've included the wp table prefix in the table creation and needed to define that in the table name identifier of the meta_boxes array but I created a table without the wp prefix and it also doesn't get data saved to it from the settings page.
I was trying to use a cloneable group with the new feature clone as multiple so just for posterity sake I tested a fieldset and it also does not save the data to the custom table so I'm thinking either the settings page does not have support for saving to a custom table or the settings page needs some special arguments that aren't defined in the documentation.