>>528352362
It's both

The metagame is everything about the game that is not directly entailed by the mechanics of the game. Which includes both what other people do and what is the most efficient strategy (given everyone isn't identical).

In games with multiple players there is no single "most efficient strategy" entailed directly by the game unless all players are identical.
For a particular match the "most efficient strategy" is a function of how other people behave.