For people like me who don't have time to watch the talk, what's the answer to the question posed on the blog post? "Why aren’t we using the Language Server Protocol (LSP) or Language Server Index Format (LSIF)?"
For LSP, the short version is that running separate sidecar services in production for every language that we want to support is a complete non-starter. That would completely eat up my team's time budget handling operational duties.
LSIF is a great technology that lets you run LSP servers in a “batch” mode. But we really need our analysis to be incremental, where we can reuse results for unchanged files when new commits come in. Language servers tend to do monolithic analyses, where every file needs to be reanalyzed whenever any new commit comes in. If you want to analyze your dependencies, as well, that exacerbates the problem. LSIF (the data format) has recently grown the ability to produce incremental data, but that requires language servers to work in an incremental mode as well. Very few (if any?) do, and because language servers tend to piggy-back on existing compiler technology (which is also not typically incremental), it will be a heavy lift to get incrementality into the LSP/LSIF world.
Whereas stack graphs have incrementality out of the box. (This was the primary thing that we added to “scope graphs”, the academic framework that stack graphs are built on.) It's the core algorithm (which is implemented once for all languages) where the incrementality happens. The only language-specific parts are figuring out which graph structures you need to create to mimic the name binding rules of your language.