Recently an important change regarding Mac firmware installation was introduced.
Apple does no longer provide up-to-date standalone firmware updaters. (Older firmwares are still available. (Note the addition of: "This article has been archived and is no longer updated by Apple.")
Instead one has to heed this advice: Upgrade macOS on a Mac at your institution:
If you're the system administrator for your business or education institution, use these methods to upgrade macOS High Sierra.
Apple doesn't recommend or support monolithic system imaging when upgrading or updating macOS.
Monolithic system imaging can only be used to re-install macOS, not to upgrade to a new macOS version. If you try to use a monolithic system image, required firmware updates will be missing from the installation. This causes the Mac to operate in an unsupported and unstable state. You can use system images to re-install the existing operating system on a Mac.
That seems to mean that the answer given here: Is it possible to reinstall MacBook firmware? might no longer be valid for those firmware upgrades (needed for APFS) in macOS High Sierra 10.13 coming within the installer for High Sierra and that they are only applied during the installation process.
How can these firmware updates extracted from the installer or the system and applied manually? Are there other triggers to apply the latest firmware on affected Macs?