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_... )
> Vertical scaling — a bigger, exponentially more expensive server
> Horizontal scaling — distribute the load over more servers
The fact that AMD chooses to package the "nodes" together on one die vs multiple doesn't change that.
> typically involving the addition of CPUs, memory or storage to a single computer.
Distinction between horizontal and vertical scaling becomes nonsense if we accept your definitions, because literally nobody does that sort of vertical scaling.