The article over-downplays the need for sentinel values. Surely Rust has Option that also spreads into C++ these days as std::optional. But for plain C using -1 or negative values to denote sentinels or error code is rather nice idiom.

The argument will be more valid if array indexes will be 1-based like in Fortran/Matlab/Julia as then 0 becomes extremely nice sentinel values. But C is C and needs -1.