The Swift VS Code extension does nothing with the semantic highlighting. All the extension does is start a SourceKit-LSP server and point at the workspace folder. The LSP server then communicates directly with VS Code bypassing the extension. If the async, await and #if etc symbols are not being highlighted correctly I would imagine it is SourceKit-LSP's implementation of semantic highlighting that has the errors. It can't be the extension as it isn't part of the equation by this point and I doubt it is VS Code as LSP was originally developed for VS Code.
I'll investigate a little more to see if I can narrow this down.