Ars is playing a video automatically when I open the page.

I thought so too, but when I load that site there's a video playing right in the middle of it.

Maybe mobile browsers consider any scrolling as an interaction or something?

I believe the rule is not to play until clicked, for videos with sound. However, for videos that don't have sound, they are allowed to play at page-load. Also, due to this functionality, sites now have a sound-less video first play and then switch to a version with sound (faking "un-muting") when clicked. This does have the nice side-effect of eliminating those really annoying ads that used to play at full blast whenever I forgot to mute my laptop's sound. (Nonetheless, I still reflexively turn my laptop's speakers down to 5% when I am not using them after getting burned so many times in public.)