They don't necessarily have to inflate their indexes. Backtick-quoted results ought to be a subset of double-quoted results, so they can use the standard quoted search algorithm, and then filter out imperfect matches from those results.

Google searches ignore punctuation, so it's not even indexed, so there's no way to search for punctuation without inflating the index

Read what I said. They can use the standard index, then filter the results as a last pass.

I did read what you said. Imagine trying to search for

  [::]

as 8192kjshad09- suggested earlier in the thread. What standard index results are you going to filter? Since "[::]" isn't in the index, you won't have anything to go on. To do your back-tick really-verbatim searches, the index has to be enlarged.

Ah, sorry, I see what you mean.