https://www.scientificamerican.com/article/edisons-system-of...
How houses are built in Iceland: https://youtu.be/7RuTizbnH4s?si=JxRjqzVFy4NLg3wa
(Spoiler; we cast them from 1 foot thick concrete slabs)
It's comfortable for the same reason as any other house: because it has thick insulation both outside and inside the concrete. See @4:30 in your linked video.[0]
If you like, sit down and do the math for how thick concrete you'd need in order to store heat for 6+ months and average out the summer/winter temperature change. It's not 1 foot thick, it's more like 30 feet thick!!
In general, the effect of thermal mass tends to be strongly overrated by people who aren't experts in building science.[1]
[0] https://www.youtube.com/watch?v=7RuTizbnH4s&t=270
[1] https://web.archive.org/web/20130509032945/http://www.greenb...