Original architectural decisions can constrain and push development in a certain direction. An analogy might be made to evolution: one's phylogeny constrains the space of available organisms available in a given timespan.

5% might not sound like much, but I'd want something comparative. How much code in the 3.8 kernel is shared with what Linus released in 1991? I suspect it's similarly low, but we still recognize them as the same project.

More important than lines of code, how much foundational architecture do they share? Code is just an implementation detail, architectural changes are more feasible in a new codebase.

I think the intended meaning was a new browser engine written from scratch, and not based on another browser engine that came before.

A similar thing has happened in Gecko. Probably modern Gecko browsers share similar amounts of old code as modern WebKit browsers share of old KTHML code (that is, very little in both cases).