We’ve noted in the firmware release note that 2.1.8 is only for OS 3.0 and OSPi. Since you have OS 2.3, you can ignore it as there is no firmware 2.1.8 for OS 2.3
In the future (e.g. for 2.1.9) you can update it using the alternative updater. The firmware update page, scroll to almost the bottom of the page, there is a link to the alternative updater, which supports all operating systems.