I'm guessing that in addition to Elixir, they also used plenty of boring technology. Like using `cron` or `at` rather than writing your own scheduling subsystem. Or building on top of a standard OSS database rather than rolling their own.
The point isn't to _never_ to use new technology, but rather to use enough boring tech that you still have mental bandwidth to pour into the new tech, innovation, or product that will be your competitive edge.