SICP only uses a small subset of Scheme, and it's only using it as a minimal vehicle for exploring computer science. You can absolutely learn the amount of Scheme used in the book as you go (that's why they chose that subset: it's not supposed to distract you from the topics they're discussing; they're trying to get as close as they can to simple axioms), but you won't really "know Lisp" (or Scheme) in practical terms when you're done, either (you'll be well-equipped to pick it up quickly, though).