No it's not additive synthesis, that is something completely different.
The O Coast, or No Coast as it also known, is referring to its design roots in the United States, lying somewhere in between east coast subtractive synthesis pioneered by Robert Moog and west coast synthesis pioneered by Donald Buchla.
They designed their gear back in the day with 2 different mindsets, Moog with his more conventional subtractive synthesis approach seen on the majority of synthesisers produced to this day, and Buchla with a much more experimental and non traditional approach seen extensively today in the modern modular synthesisers for example.
A traditional west coast synthesiser didn't contain any filter modules, as apposed to Moog's design virtually built around his legendary filter, hence the term subtractive, this filter subtracting from the sound source.
The O Coast is supposed to be designed in between both camps, the best of both worlds in a very small footprint. Really cool synth with a lot of power for its size.