It was sold as a separate game. "Today we are very excited to announce the launch of a new stand-alone product, Elite Dangerous: Arena" https://community.elitedangerous.com/en/arena. They marketed it as a cheap £5 ED-lite than could be "upgraded to the full galaxy". They then game it away free and shortly after withdrew it from sale.
Even long before that, it was available from the main executable as a fourth game mode. (And still is, of course).