One point the authors make is that registers are memory. I’m not sure they’re saying screw registers, just that they’re counter to portability and in general only a yet faster cache for programs to use. The other point they seem to make is that a secondary reason we have architected registers is that renaming over an arbitrarily large space was (and in some ways is still) difficult. By using graph coloring and reg allow in compiler, we narrow the encoding space before execution and give the hardware a much simpler problem. Recent research results on register less arch show that there are quite a few benefits. There’s a lie in calling it registerless though, it’s still there, just transparent and hence portable.