It may be using some of the same deserialization machinery, but "parsing" is a broad term that includes things that the sanitizer is doing and that the browser's ordinary content-processing → rendering path does not.

Even with this being a native API, there are still two parsers that need to be maintained. What a native API achieves is to shift the onus for maintaining synchronicity between the two onto the browser makers. That's not nothing, but it's also not the sort of free lunch that some people naively believe it is.