That would likely only muddy the waters further, as that's a thread about Lightning, while the confusion here lies with SegWit.  While people tend to refer to them in the same breath whilst talking about improvements, they are very separate things.
  


Reading all the BIPs might be overkill, but on the other hand, I find the vast majority of clips about Bitcoin on YouTube completely unwatchable.   Anything by Andreas is usually a safe bet, but again, that particular link is an older video, so I don't know if there's a more recent one that's worth checking out.  The Segregated Witness wiki page might be worth a shot too.

The easiest way to demonstrate that SegWit has indeed been activated is to simply look at a list of the latest blocks being produced, whether it be XBT.eu or any other such site.  You can plainly observe that the vast majority of blocks being produced are over 1MB in size, something that would never have been possible prior to the activation of SegWit.

If anyone is still using software that doesn't support SegWit, that software won't see as plainly as you can that the blocks are larger than 1MB.  They'll still see that every block is less than 1MB because they simply ignore the data in (what should be correctly referred to as) the new "witness space" (which I assume is what they meant by "block extension").  In effect, some nodes will now read the same blocks differently to others, depending on whether they support SegWit or not.  When you send a Legacy transaction (i.e. not SegWit), all the data, including the signature data goes in the "base" 1MB space and every node can read that.  When you send a SegWit Transaction, the signature data gets to take advantage of the extra witness space, in order to fit more transactions in the base 1MB, but only SegWit-compatible nodes will recognise them.  This means we can effectively increase the blockweight in a backwards-compatible and entirely voluntary way.