I don't think it's either. "Scaling" refers to growing the amount of compute power you are using. This change falls into a different bucket, which is making more efficient use of the compute that's already available.
If they are running multiple Docker containers, the improved efficiency would allow them to run more containers per host computer. Maybe that's what they meant, although it's not really horizontal scaling either.
I think scaling is more about serving demand - whether you you use more resources on a single box, more resources by using more boxes or use the same resources more efficiently you're still scaling.
This is the closest to my use of the term. I'd go further and say that using more resources on a single box is vertical scaling, using more boxes is horizontal, and this thing is neither. How about density scaling?
If they are running multiple Docker containers, the improved efficiency would allow them to run more containers per host computer. Maybe that's what they meant, although it's not really horizontal scaling either.