It doesn't prevent every falsification. It makes it much harder.
Say the incident is a car crash. And assuming dash cams also get this blockchain feature. The car crash already happened and you cant fake a video afterwards that make it look like the other person hit you first. And if you try to preemptively fake a video, you cant know every possible roads, roadsigns, nearby cars, or what vehicle the other person is driving, basically you cant predict everything that was on the scene before the incident occurs.
Imagine if you hit a truck at 5 PM heading west on Road 27 and on the intersection on Road 52. You'd have to know beforehand the road that the incident will occur on, the position of the sun (its 5PM and you're heading west, remember), the road signs, how wide the road is and how many lanes, the fact that the other vehicle is a truck, what the truck looks like, etc.
I mean you have to create so many fake vehicle collision videos then when an incident happens, you'd have to hope one of the ones you faked matches the situation, then quickily find the video and send it to the blockchain.
Not to mention, the other person could have a dashcam video without any discrepencies. And any slight discrepency on your faked video would make court believe the other recording more than yours.
I mean its not impossible fake something. But its hard to do it before something happens.
For niche things, you kinds have to go to reddit.
I mean the worst of reddit is on mainstream topics like politics anyways. You're less likely to see toxicity in like a gaming subreddit. (Less likely than politics anyways)