This practice (adding a ? to a declarative sentence) is very informal, and so you wouldn't see it used in published English language media, so I'm not surprised that you may never have encountered it. It's used as a shorthand method of inviting clarification when the author is unsure of the veracity of the claim.

This sub-thread reminds me that there are many informal rules in languages that are rarely taught, and which can trip up non-native speakers. Informal shorthands, idioms, slang, etc.