ADB Sideload needs so-called OTA files. The install files currently offered on the download pages aren’t OTA files (they partly were for a while in the past … the upgrade guide is now outdated, the currently available files match the manual install guide).
However, https://ota.ecloud.global/api/v1/FP3/dev or https://ota.ecloud.global/api/v1/FP3/stable will give you links to the OTA files (see the “url:” lines).