No, it's valid advice. Non-WMO starts a frontend job for each file, WMO runs one job.
The frontend jobs do not share state, so each one parses all the files in the module.
In general the parser is very fast, and the type checker tries to only type check declarations in files other than the primary file when absolutely necessary, so it's not always O(n^2). But there are pathological cases you can construct today where the type checker ends up doing too much work.
`swiftc * .swift` spawns one frontend job per file and then runs the linker to link together the .o's. `swiftc -whole-module-optimization * .swift` compiles all files in a single frontend job. Note that -O is independent of -whole-module-optimization, which is perhaps a bit confusing.
