Undefined behavior is considered worse than crashing, which is typically the alternative when reaching "unreachable" codepaths.

Compare these two blocks similar to the article

   switch (ch) {
   case 'a': do_a(); return;
   case 'd': do_d(); return;
   // ch is guaranteed to be 'a' or 'd' by previous code.
   default: assert(0);
   }

   switch (ch) {
   case 'a': do_a(); return;
   case 'd': do_d(); return;
   default: std::unreachable();
   }

If the programmer is wrong about `ch` in the first one, the program terminates. For the second one, the compiler could change it to be equivalent to

  if (ch == 'a') { do_a(); }
  else { do_d(); }
  return;

If the programmer is wrong here, the program might `do_d()` with unintended consequences. I'd say "going down unintended codepaths" is typically considered worse than crashing.

N.b. fixed last code example- thanks afiori.