I was actually about to make a comment on that. They got that number from some dumb "experiment" they did where told two groups of developers to implement an HTTP server in Javascript. The group with Copilot got the job done 55% faster (https://github.blog/2022-09-07-research-quantifying-github-c...)

So yeah, they did an experiment with <100 developers asking them to create implement something that only took the control group 3 hours to finish from scratch, and from this we got the "55% more productive" statistic.