Information about whether a native wikitext construct or a HTML tag generated a HTML dom element is currently recorded as part of the "stx": "html" flag in data-parsoid. Data-parsoid is considered private information and is not exposed to clients (while they are free to inspect it, there are no guarantees that the spec and format of it won't change without notice). However data-mw is public semantic information.
This HTML syntax flag seems like semantic information as far as serializability is concerned. For example, there is no way currently for clients to specify that Parsoid should generate a HTML tag during serialization since data-parsoid is not something they should be messing with.
This also matters for https://gerrit.wikimedia.org/r/#/c/208993/ where we are currently blanket-stripping data-parsoid but this means that some information is lost where the subst-ing produces HTML tags.