Using mosh with SSH jump hosts

I have a reverse SSH tunnel setup between bigbox and jump so that I can login into bigbox from where ever (it is behind a firewall) by using jump as a middle man. This works fine: I just ssh bigbox from my laptop and presto, I am in. Now, I want to use mosh to reduce lag, but I am not sure how to get it working. I have no experience with Mosh from earlier. I basically want to use SSH to setup the initial connection using my configuration and then leave it to Mosh. Unsurprisingly, neither doing mosh bigbox nor mosh --ssh="ssh bigbox" localhost did work:

$ mosh bigbox
/opt/homebrew/bin/mosh: Could not connect to localhost, last tried ::1: Connection refused
kex_exchange_identification: Connection closed by remote host
Connection closed by UNKNOWN port 65535
/opt/homebrew/bin/mosh: Did not find remote IP address (is SSH ProxyCommand disabled?).

Here is my config: # ~/.ssh/config

Host jump
    HostName jump.somedomain.com
    User jumpuser

# https://unix.stackexchange.com/questions/693885/hardening-reverse-ssh-tunnel-via-jump-host/693886#693886 
Host bigbox
    ProxyJump jump
    User my_user

    # Using a reversed SSH tunnel
    HostName localhost
    Port 20001