How to update the EFI firmware of a replacement logic board of a MBP mid-2015 to make it compatible with Mojave?
Let me outline the solution here in case any one else encounters the same problem.

The installer wouldn’t allow me to select the internal flash drive of my MacBook Pro, and the precise installer error message was “This Mac needs a firmware update in order to install to this volume. Please select a Mac OS Extended (Journaled) volume instead.”

So there were 2 solutions:


Backup the internal flash drive, erase, reformat as HFS+, and run the installer again.
Install Mojave on an external HFS+-formatted drive.


Explanation: with the “stock” firmware of a replacement logic board, the installer expects an HFS+-formatted drive, and won’t update the firmware until it can install the OS on an HFS+-formatted drive.