I disagree with their statement that individual developers do not pay for tools. I have paid for tools out of my own pocket on many occasions. However, being able to deliver code 18% faster isn't enough to fork out $9.99 a month. First of all it is relatively expensive. For that amount I can get a personal license for PyCharm. Secondly coding speed never tends to be a bottle neck for delivering a feature or a product…
This metric seems silly on its face. 10 bucks to get 18% more productivity out of a $10k per month developer? If this was indeed the case, everyone who employs software engineers would instantly pay this. Maybe they should have marketed more? Or maybe there are other problems with the technology (e.g., fears over copyright infringement?).
We have plenty of techniques that we know improve productivity (e.g. static types) but some people still don't believe it because it's really hard to prove productivity increases.