Might as well ask what's indicative of stone tablets from millennia ago being data to us now? These things aren't discovered and studied in a vacuum. They operate within context - where the items were found, their similarity to other better understood things, known history of data storage, etc etc.

Given enough time and disruption, sure, all context could be lost, but if that's the case, I'd assume figuring out what the weird glass cube thing is would be the least of their problems.