I struggled with how to word that in a way that's both true, understandable, and doesn't give away any proprietary information. Added "indexed" to clarify but I didn't fix up the numbers, so they're likely an overestimate.

Basically, yes, it uses an index and touches only documents that appear in one of the relevant posting lists. However, after stemming, spell-correcting, synonyms, and a number of other expansions I'm not at liberty to discuss, there can be a lot of query terms that it needs to look through, covering a significant portion of the index. Each one of these needs to be scored (well, sorta - there are various tricks you can use to avoid scoring some docs, which again I'm not at liberty to discuss), and it's usually beneficial to merge the scores only after they have been computed for all query terms, because you have more information about context available then.

There's a reason Google uses an in-memory index: it gives you a lot more flexibility about what information you can use to score documents at query time, which in turn lets you use more of the query as context. With an on-disk index you basically have to precompute scores for each term and can only merge them with simple arithmetic formulas.