I'm sure it can do a plaintext search just fine. What the author is talking about is language-aware features like "go to definition". Holding all of a whole web browser's C++ parsing tree in memory is a lot bigger ask than just its plain text.
You're assuming that the only way to a definition of an identifier is 1) parse the entire source tree 2) keep the entire source tree in memory 3) use that in-memory source tree to go to definition. If you accept those constraints and then implement in a slow language, then yes, it won't work.
You’re rather underestimating the vast expanse that is the Chromium codebase, I think. That said, distributing tags files was a common thing once, and a dedicated symbol package you could just download and feed into your language server (instead of being constantly tethered to a symbol server) could make for a nice affordance today.
So this actually exists, as it turns out. You can run clangd-indexer to produce a static index and then load it with clangd -index-file. The caveat is that
> [r]unning clangd-indexer is expensive and produced index is not incremental.
