> Moreover, the experience can be completely asynchronous. You don't have to both be available to make progress.

Studying SICP (or anything) can involve different "modes":

Reading and doing exercises will often be solitary. Q&A could definitely be asynchronous (IRC works great for that!). But we're talking about explaining what you've learned; only in synchronous conversation will you get the back-and-forth "wait what? How's that work?"

A hard part of learning is figuring out exactly what you don't know; conversation is remarkably effective at elucidating that. Where do you feel uncomfortable while speaking? Where does your partner look especially confused or skeptical? The human connection gives you expression, body language, tone of voice, etc. in a quick feedback loop.

(disclaimer: not involved in creating Understudy, but I'm a happy user!)