You need pipes to spread it out. And I think you may be underestimating the scale and force of ocean currents. Every tide probably moves more water than California uses in a year.
Crucially but overlooked so far in these threads, is that the proposal does not have brine exiting on the California coast; it has it flowing into the Sea of Cortez.
That should be an obvious non-starter for anyone remotely familiar with the unique geography and ecology of the area.
On average it's pretty close, but you can definitely get impacts from increased local salinity: https://onlinelibrary.wiley.com/doi/full/10.1111/gcb.16859 (just random ecamples)
https://www.scientificamerican.com/article/slaking-the-world... (view in incognito)