I think the part "if you want the WebView UI component to be consistent across platforms (you want this)" is a reference as to why a built in webview won't be good enough.
Browsers themselves don’t do this for security snd sandboxing reasons. Each tab is another instance.
Each tab is a partial instance. There are still many resources that are shared globally for performance and memory use reasons.
But operating systems are free to map the same read only memory pages (e.g. program code) into different process address spaces. This is an optimization that running processes are generally completely oblivious of. But it only works when e.g. the program code is mapped into memory from the same file. If every program has a private copy of the same library (even if it's bit for bit identical), the operating system doesn't have enough information to pull this off.