> Is only 10 people representative of the population of thunderbird users?

For very narrow studies it is possible to get representative data with fewer than a dozen interviews, but in this case it is explicitly not representative. In the video they mention that most of the participants have used Thunderbird for over a decade and follow release notes, development, and various forums closely, which to me suggests that they were recruited opportunistically rather than a random statistical sampling.

They do mention that they have plans to engage a larger audience in the future but that can be incredibly expensive. Even large organizations typically have to augment a small number of representative interviews with a large number of surveys and a very large set of user telemetry to properly weight interview feedback.