LSP is probably the best way to provide the fixed set of classic IDE features that LSP does - inline diagnostics, autocomplete, go-to definition, etc.
We think that for many other features, Tree-sitter is a much cleaner solution than pushing the logic out into the language server. For example, if you want accurate syntax highlighting that's lightweight and updates immediately (as opposed to in a delayed fashion like in most IDEs), you need incremental parsing. Incremental parsing is a highly specialized problem, so writing an incremental parser in the form of a Tree-sitter grammar is much simpler than modifying a language's compiler toolchain in order to make it incremental, and bundling that whole toolchain into an app like Atom.
The same goes for other features besides syntax highlighting. The syntax tree is now available via a uniform, in-process API in Atom, and is always up-to-date, so you can script the editor to manipulate your code intelligently. I'm not totally sure what kinds of things we'll end up building on top of that, but I think it's a different set of things than LSP will end up providing.