So it does look like the TypeScript language server has a limit of 4MB source size where it disables type checking (and actually shows an erroneous error stating that exports that exist in the file do not exist) for files that are imported but not open in the current workspace/session.
Still not sure if this is documented anywhere or not (haven’t been able to find it, if it is).
99.99999% of the time, unless you’re doing niche stuff like I am, you won’t run into this.
Workaround: should you have such a large file, e.g., with a large generated object, try and refactor to split it up into multiple files and rejoin it a separate file. The actual object size/memory usage isn’t the issue, it’s the file size.