Those that already read the "What If?" section might want to jump to 7:10, which is when he explains what happens after he publishes the article about punchcards[1]. I mean, it's a nice talk, and if you haven't read the article then totally watch it, but as I already read it...
They have a lot of data, which means they have a lot of data to back up. Tape drives are still the go-to method for backing up huge amounts of data.
for paranoia reasons i thought they were using it for permanent storage.
maybe they are using it for some sort of rolling backup.
Rolling backup is at least one use case. This is why they only guarantee data destruction after 180 days (at least per the last document I read about it) for Apps customers.