Outside the world of foss there are for example multiple games that are highly respected that were all entirely or largely developed by a single developer. Rimworld, Songs of Syx, Dwarf Fortress, Ostranauts and many others. In a proprietary context I have myself deeply understood codebases and worked with others who have also.
That being said, you are arguing against a straw man that neither I nor the rust article propose. They are not saying maintainers need to understand all the code they are saying the person submitting a pr needs to deeply understand that pr. Why? Because they want to build a community who deeply understand various aspects of rust so when things need maintenance there are experts around who even if they don’t know a particular piece are invested in the success of the project and know enough to get up the curve and fix whatever needs to be fixed.