I'm assuming the parent means that just running the steam client switches over to discrete graphics, which would severely impact battery life for no real performance gain.

Also, some games play fine on the integrated graphics, so switching over to discrete isn't necessarily something you always want.

I think they meant it's triggered by the presence of Steam itself, not a game you might launch from it. Steam takes the liberty of adding itself to your account's Login Items.

Though I'd guess the workings of this auto graphics switching mechanism are really Apple's responsibility.