Consider a uniform repetition approach that repeats each item once per day. The rate of our uniform repetition system scales linearly with respect to time. If one item takes 1 second to review, then I will lose 1 second per day. Therefore, N items causes me to lose N seconds per day. Even in the case where I don’t add items to my list, I lose N seconds per day.
In contrast, as you stated, the amount of time we lose per item per day reduces exponentially with spaced repetition, however. If we don’t add any items to our list, the review time asymptotically approaches 0 seconds/day.
Unrelated to all of this, I agree the title may be a touch embellished. :)
The first rep gets you 1 extra day of knowledge. The next rep gets you 2 extra days. The next one gets you 5 extra days.
So each rep is getting you more and more knowledge even though the time required to execute the rep remains the same. This is exponential growth.
It's like someone giving you $10 and then trying to argue that they haven't actually given you $10 because just a few minutes ago you had a $10 bill in your pocket but you spent it on something else.
This article talks about how the reps needed to learn one piece of information, reduces exponentially over time. You might need 1 rep per day at the beginning, but only 1 rep per 100 days after a month. This basically means that if you have a lifespan of 100 years, spaced repetition means you only need around 10-20 reps to remember each piece of knowledge for the rest of your life.
But learning N items will still take 10*N reps. It scales linearly. A far cry from exponential