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.
The author addresses this, you can get explanations that seem logical, but ChatGPT is not telling you what it was actually "thinking" about when generating the responses. It's just looking at what it's said and coming up with an explanation after the fact.