It doesn’t work for me with the /e/ recovery image:
$ adb sideload e-0.16-q-20210414110657-dev-gemini.zip
adb: sideload connection failed: closed
adb: trying pre-KitKat sideload method...
adb: pre-KitKat sideload connection failed: closed
It works when using TWRP, but then still e-0.16 boots but is not usable. I’m guessing due to a write permission problem? Not sure. But it sideloads with TWRP, and with /e/ recovery it does not.