No, he says most people do not actually implement the interface to make it make sense. So they have to make ctrl-z and ctrl-shift-z do the logical linear thing

No, he says most people do not actually implement the interface to make it make sense. So they have to make ctrl-z and ctrl-shift-z do the logical linear thing which requires all sorts of craziness which is what is complicated to do correctly (e.g. without breaking user expectations).