Previously, if cities did upzone, a neighbor could still sue their neighbor to stop them from building. In places like SF this means every project is tied down for years and hundreds of thousands of dollars of legal fees.

Now, you are effectively not allowed to do that. So the permitting process to build a duplex is as straightforward as the permitting process to build a SFH.

Which, to be clear, is still really hard in most places in CA.

Definitely!

Someone else could explain better but I'm also fairly sure many of the new rules here (such as exempting CEQA) also apply to SFH. They just leave the current rules untouched for commercial/large-scale residential.

The idea with SB10 is that, once the next RHNA cycle starts and cities are required to zone for their share of housing (or face punishment), they will use SB10 as an "easy way out" to upzone while skipping a lot of the usual red tape (CEQA, etc).

Under RHNA and the HAA, if cities fail to zone for their share of housing, many projects will be approved automatically and the city loses its discretionary review authority.