Can't help but also think "WTF are they doing there..." - we're doing exactly the same (user segmentation, targeting and campaign execution for cinema & movie users, disclaimer: we're more or less their only competitor, albeit indirect), but our solution is running at ~30k/year total at 10 times their user base. No magic in there, just good architecture and solid Computer Science. Boring technology (Go/Redshift/Postgres/S3).

The only thing I'd fully agree on is that using Go saved us a lot of resources as well. It's an awesome choice for stuff like this that needs to be reasonably performant as well as being simple, understandable and reasonably fast built.