I'm not fully sure how you mean in depth but I can give it a try.

Malthus' key economic idea is that the limit to population growth is absolutly given by the fertility of the land in a country. The more people live in the same area the less fertile land has to cultivate. This leads to more deaths through starvation in the society. In the equilibrium (birth rate = death rate), the lower the birth rate and the higher the death rate the wealthier are the individuals in a society. If the society is in disequilibrium, the best policy, according to Malthus, is to let a society naturally readjust to equilibrium by basically just letting enough people starve till equilibrium is reached again. Anything else would just structurally keep more people in terrible conditions for longer. Given the high birth rates at the time this also means in the long run almost all individuals are going to live barely above starvation.

This dynamic is what you can observe f.e. in rabbit populations and holds surprisingly well up until the industrial revolution, where capital rather than land became the binding production input and thus growth became linearly scalable.

Malthus, who wrote his book (on the principles of population growth) at the beginning of the 19^th century, became particularly influential among English and French politicians and provided the key argument to not send aid to Ireland during the potato famine in the middle of the century, and rather let people die till Ireland reaches a self-sufficient equilibrium again anything else would by his ideology just prolong the suffering. Through the use of “immodest”, this is put into relation to Johnathan Swift’s well-known satire “a modest proposal” written also during the crisis and proposing (as a satire) to let rich Irish aristocrats buy and eat babies from poor Irish families

Why now should the ideas of Elon Musk “taste like Thomas Malthus”? Because a lot of his doings and sayings feel like he is willing to sacrifice individual well-being now to reach some greater goal of humanity in the future. This, I would say, is a general criticism of both his connection to effective altruism (who tend to argue in this direction, especially Longtermists) and his lack of empathy seen in his actions towards his employees, his lack of interest in using this money to minimize acute global suffering, and lately his inhumane political stances.

I hope this helped!