That is not 84 hours with the buds, its 84hours of charge available in the case.
The buds themselves only last maybe 8 hours or whatever, not sure the exact value, but then you can put it back in the case and wait for it to fully charge again, and it can charge the buds ~10 times before the case needs charging again. That's how they get this 84 hours value, its a bit misleading.
I have the Anker Life Dot 2, it says 100hour play time, but that with the case. The buds themselves only last 8hrs continuous playback