I disagree with this. If this was true, boxer engines would be blowing smoke everywhere.
Not necessarily so.
Have you taken the oil sump configuration into consideration in your comments?
Typically an automotive boxer engine configuration will have a much lower sump and comparitive oil level from cylinder centerline versus OPE plus the addition of pressurized lubrication working in it's favor.
A splash lubed OPE application is much more likely to emit a puff of smoke due to the relative level of the oil sump and the likelihood of oil pooling in the cylinder bore at shutdown.
Now I don't necessarily agree that most
horizontal piston OPE will smoke at start-up, but it certainly is more likely.
On top of that, the customer base is more willing to tolerate the puff coming from a lawnmower and not complain to the OEM whereas in an automotive application customers are very sensitive to this kind of performance and will not tolerate it.