What proof could they actually provide? Given the medium, anything they said is probably easily forgeable or illegal to share. That being the case, demanding proof is just a way for you to look "skeptical" without providing much of value to the conversation. I'm guessing you meant well, but apparently others thought differently.

We all know people lie on the internet sometimes. We can't explicitly bring up the possibility everytime someone makes any statement, or half the internet would be "careful, this might not be true!".