I recognize just now that Debian builds Dropbear without X11 forwarding enabled. I actually does support it with the respective variable set at compile time: dropbear/src/default_options.h at bd12a8611b3c838f1ed1d1c2cbaff2da1072a315 · mkj/dropbear · GitHub
Oh, has actually been discussed here already, only compile time options have been confused with runtime options 
: Dropbear X11 Forwarding on Mac with Quartz