Lets note that 95% of small businesses make less than 250k.
If a business spends $800 and makes $1200 that is great. The risk is small. If the same business spends 8k to make 12k that is great. The risk is bigger but relatively small. If the new hired gun, self-proclaimed-marketing-guru, decides to jack up ad spending to 80k to make 120k, someone is betting too high now.
Asking if this model is scalable is bringing emotion into this scenario which will guarantee you failure sooner or later.
At one point you will reach the amount at which spending more on ads will not increase your revenue. I guess that is when justifying "branding" comes.
the article is analyzing "cost of acquisition" relative to revenue. As noted in other replies, this is not including total operating expenses. So for the business, the actual net profit margin is going to be lower than this.
a 50% ROI seems pretty good compared to traditional businesses