When I lived in California (only 40 plus years) there were great differences between slang in various parts. We had all sorts, it's a big state. Can't say I heard boss much but spent most of my time in northern california, which was fairly different from southern california, especially the cities vs sticks and all that....
LOL urban dictionary has this from 2010 for that's boss:
Used as a way of saying something or someone
is stupid,
ridiculous, and gay. A form of replacing the word
that's gay. When someone says "that's boss" they are referring to something or someone that they do not like
(never heard it used in this respect myself)
Then in 2015 its
That's
awesome!!
Hey man,
I'm going to go have a few drinks and then hit some
surf!
-that's
boss!!
(this is more what I heard in socal)