The command line install instructions given at Install /e/OS on a Fairphone FP3/3+ - “FP3” are currently only valid for the stable install files, not for the dev install files.
The stable install files currently come in the format which installs partition images with fastboot, the command line install instructions cover this.
The dev install files currently come in the OTA format ready for use with ADB sideload in the recovery, the command line install instructions don’t reflect this at all as far as I see.