Reddit and Twitter starts video with muted sound on my browser (Edge).
My guess from someone who had to develop a web video player at work, many websites will attempt to autoplay the video with sound and if it fails, it's easy to catch the failure event, they will mute the video and try again.
I'm talking specifically about Chrome. There's no web standard that says what a browser must do about autoplay requests, and Chrome permits a large number of sites to autoplay with sound on.
Web browsers are also capable of determining that autoplay on technically-not-load-but-automatic counts as autoplay. (There's even text in the spec about it.) In particular, they can tell whether it is in response to a user action/gesture on the site or not.
Chrome has some special logic about autoplay. The following page describes them, but I feel like it's a bit more complicated in more recent versions of Chrome.
My guess from someone who had to develop a web video player at work, many websites will attempt to autoplay the video with sound and if it fails, it's easy to catch the failure event, they will mute the video and try again.