This is in practice not true at all. Vertical scaling is typically a sublinear cost increase (up to a point, but that point is a ridiculous beast of a machine), since you're (typically) upgrading just the CPU and/or just the RAM or just the storage; not all of them at once.
There are instances where you can get nearly 10x the machine for 2x the cost.
(and fwiw, wikipedia agrees with this definition: https://en.wikipedia.org/wiki/Scalability#Horizontal_(scale_... )