Guido can vote; he can also Pronounce. -100 is a shorthand idiom for "veto".
(I know you were joking, but I figure there are probably people reading this who are not familiar with the Python development process or Guido's role in it.)
To further elucidate for those people, the +/-n numbers are not really votes -- nobody is adding them up and making a decision based solely on the total. They're just an expression of opinion, so you sometimes see things like -100 or -1000 to mean "I *really* don't like this".