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https://twitter.com/TwitterSupport/status/1682501248334061572
A tip isn't a story in itself, it's a suggestion of where to start looking for one. "Hey, I'm off the record here, but some shady shit is going down at my job and you might want to fire off a Freedom of Information request for x, y and z or talk to a and b" is a pretty good tip.
Guess you're right, I just find it weird for anyone to leave a worthwhile tip over an unencrypted third party company's messaging. It's like "I'm off the record... but I don't really care who hears me say it".