The thing about configs vs C is that many apps have their own config conventions, they deviate a lot from one implementation to another and you have to study their documentation (if it exists).
C on the other hand is one standard, some things differ in conventions in libraries, but most standard ones hold to one convention and you may apply solution from one adaptation to different suckless programs.
Of course it requires knowlege of C and the technology application works on, but that’s not entirely true.
Thing is while other configs have their own documentation, C language has many books, standards, articles code snippets et cetera, which is C’s documentation (sort of).
I guess it is inherently hard to understand for regular individual that while configurable apps explicitely direct you to the config documentation, suckless software doesn’t do that with directing to C books because that should be merely obvious.
Suckless software is definitely not for all and it isn’t universal (in some sense) but that’s how it can stay simple, elegant and maintain small codebase.