[Tervuren]

I was just wondering why use the separate additional descriptor opposing if not to disambiguate.

The way I parse language I'm wondering why take the extra time to expressly mention opposed.
If not to convey the pistons moving in opposition vs together, what then?

Any production horizontal engine that I am aware of just saying it is flat or horizontal is enough descriptor to convey the engine layout as having the cylinders opposite in common parlance.

And speaking of inline 6's, maybe some one made a flat inline engine back in the day.
I know Indy cars of the past really leaned the engine over for balance and CG. Not having the drive shaft run down the center of the car had the additional benefit of lower fuel tank and driver placement.
A short period of Indy had FWD dominated machines as it lowered the CG by not running a drive shaft down the chassis at all.