Generally, browsers try to make it very difficult for the contents of one website to read or interact with the contents of another website. There is a class of attacks called cross site request forgery (CSRF) where website A tries to trick the browser into sending a web request to website B and performing some action which requires authentication. In this case, the action would be to like a Facebook post.

Imagine something really basic like your bank has an endpoint GET shite-bank.com/account/transfer?funds=100&to=myEvilAccount. Website B could try and redirect you to that URL. If you're logged in to shite-bank, then when that request completes you will transfer 100 funds to me. Generally, most websites use various techniques and tokens to prevent other websites from triggering requests like this.

I clicked through the source article, and it sounds like this is specifically a windows thing: original article says that when a windows machine loads the malicious SVG, the malicious JS is parsed in a Microsoft Edge browser process, regardless of the browser a user visited the porn site with (apparently all Windows SVGs load through Edge). I would guess that there is some aspect of this context switch which enables the CSRF attack to work, but it is not explained in the original article.