Is time-to-last-byte really defined as the moment the initial response has been successfully received? Or is it defined more vaguely as "all responses needed to

Is time-to-last-byte really defined as the moment the initial response has been successfully received? Or is it defined more vaguely as "all responses needed to render the page have been received "? I'd understand "time-to-first-byte" as something that could at least give you a reliable lower bound for how long users have to wait, but I don't see how "time the initial response has been received" is any kind of meaningful indicator of UX in modern javascript-driven websites.