The most common trajectory for react and other SPA framework apps is to also make this assumption, waving away the weight of libraries and front-end business logic with talk of how build tools are stripping out unused code so it must be light, while frequently skipping affordances for outright network failure that the browser handles transparently, oh and hey don't forget to load it all up with the analytics calls.
But maybe more crucially: what's the real difference between the overhead of generating / delivering markup vs JSON? They're both essentially data structure -> string serialization processes. JSON is situationally more compact but by a rough factor that places most components on the same order of magnitude.
And rendered markup is tautologically about the data necessary to render. Meanwhile JSON payloads may or may not have been audited for size. Or if they have, frequently by people who can't conceive of any other solution than graphql front-end libraries.
Whether you push html or json or freakin' xml over the wire is a red herring.
Heck, "nativeness" might be a red herring given frequent shortcomings in native apps themselves -- so many of them can't operate offline in spite of the fact that should be their strength because native devs ALSO assume client's internet is fast/on.