The article was updated in 2017 so the information should be current for 2015 models. I am familiar with Oakely's site and free apps. You can always ask him about it. I have no idea how the error is being generated. All Mac disks have and EFI partition that was once used for boot code but is no longer used for anything. Nevertheless, there does not seem to be firmware information about the 11,4 model. I have checked another source which showed no update.
Both Mojave and Catalina installers will do firmware updates as part of the installation process such that it is out of the hands of the user. Have you checked the firmware versions using System Information? Have you called Apple Support for assistance?