It's not that people are idiots, but that threads are always including information that is incorrect but has been reported so often, that believe it to be true.
And yes, people should actually check out the TV licencing website and Gov. site but many, many people don't and MN and many MN posters are constantly telling the public and posters what a fantastic hive of education and information it is so incorrect info gets repeated and becomes fact when it isn't.
And also, you're correct you don't need a TV licence just for having a TV, but only if you use certain services but you do need one even if you don't have a TV at all but use those services from a laptop or even a 'phone or any device.
Because that's another myth perpetuated "I don't have a TV at all, just watch stuff on my 'phone or laptop so don't need a TV licence".
And yes, you're right it applies to BBC iplayer and watching live TV on any service but it also applies to recording live TV on any service to watch later. Because that's another myth "we don't watch live TV, DH records the footie on Sky to watch later so we don't need a TV licence" - TV licence still needed.
And also applies to watching anything live on YouTube, but not to pre-recorded videos. A lot of people think TV licencing doesn't apply to YouTube at all, but it does if you're watching "join us for a live q and a at 8pm" - TV licence needed.