The table operations online operation executes as a fate operation. If a transaction lock for the table is currently held (say by table compactions) the online operation will block.
This modification essentially changes the behavior of the online operation to a NOOP if the table is currently online. If the current table state is online, then the operation returns immediately without queuing a fate operation to set the online table to online. This eliminates the blocking behavior of the operation if the table is already online.