They're just storing doubles in their own format too.
I'm not sure if they even need any spatial lookups on the data, they didn't mention anything about that in the article. Maybe they do that in-memory?
I imagine a delta encoding scheme similar to what the time series DBs use would work well for the geo points. Maybe even a delta-of-delta encoding for things like ships which move very consistently.
It's probably not worth it given how small they've already go their data. But it is fun.