Yes, but "share-nothing" almost always has some sharing. (If it doesn't, it can be run somewhere else, by someone else, at some other time, including "never".)
They have a common document cache, a common cookie store, a common history store, probably a common dns lookup result cache, and maybe some other things. These all need to be carefully synchronized between multiple tabs.
In fact, I would argue that multi threaded would be better than multi process so that even more things could be easily shared. For example, I imagine that css stylesheets are parsed into some big fat data structure inside browsers. If I open two tabs from a website that share a stylesheet it would be optimal if they share this same internal representation (no locking would be required for the sharing since it's read-only). This has the obvious savings of memory, but it also increases speed since the css file only has to be parsed and processed once instead of multiple times. And things like sharing keep-alive connections between tabs are virtually impossible with multi process, while very possible with multiple threads.
Uhh, from the OS they get that for free.
But don't undervalue the difference in mindset. At least for me, threading makes me think "what can I peel off of my main task to run in threads" versus share-nothing which makes me think "here are the 3 shared resources that I anticipate will be bottlenecks".
Expensive lines make systems less flexible, not more. They lead to copies for efficiency, aka denormalization, which is another word for "bug waiting to happen". While this can be dealt with, doing so involves lots of nasty tradeoffs, with bugs a common outcome.
More to the point "more modular and more flexible" is neither necessary nor sufficient for producing good software.
I'm glad I'm not the only one that thinks this way. I wish I could upmod by 1 million.
Don't do that.
> "here are the 3 shared resources that I anticipate will be bottlenecks".
Do something like that.
The difference between threads and processes is in the mechanisms for sharing. While those mechanisms affect how you organize computation (different things are cheap), they don't mandate an organization.