In the case of Reddit, somebody posted YouTube videos on a web page without notice, as illustrated by Android Police and 9to5Google. You only need to add an additional period in the URL for any video you like to view in the YouTube link after the com dot.
This works as I have described when you paste a video URL with the added time into the Incognito browser window. It also removes interruptions for videos as well as the elimination of pre-roll advertising.
I am not certain how much time you really save on waiting just for the "Skip ad" button to appear when you copied, pasted, and change the video URL, but it really works.
As for why it's too trivial to get through Youtube's advertising system effectively, this is what Redditor unicorn4sale said when posting the trick.
Websites fail to normalize their hostname, content is still being served but there is no corresponding hostname on the web server, no broken CORS cookies and even larger Websites use a separate domain in which ads / media can be provided with a whitelist which doesn't include additional dot
With more than 4,000 Reddit upvotes, It's probably already known that YouTube has taken wind from this gap. Probably sooner rather than later the monitoring will be corrected.
In addition, remember that your favorite creators partially help with their ad revenue. Ad blockers and this trick is taking the money away.
You should be aware that the "proper" way of not seeing advertisements is by subscribing to YouTube Premium, since the YouTube application constantly bannes people with crazy frequency about this.