Another factor is the location of the study in Sweden. In summer the typical peak UV index in Sweden is 5 whereas in Australian summers the UV index is often 11+.
If you compare protein per kJ instead, broccoli has 0.021g protein per kJ whereas lean beef mince has 0.028g per kJ. Much more similar. Although of course you would need food that is higher density protein as well so you don't have too much volume to eat.