How to reliably block autoplay?

There's already a decent help article about how to block videos from autoplaying: https://support.mozilla.org/en-US/kb/block-autoplay The problem is, it works haphazardly. Even if my default for all web sites is "Block Audio and Video", on some web sites (not all) embedded videos still start playing as soon as they scroll into viewport. Some have adviced to delve into about:config and set media.autoplay.blocking_policy to 2 (default is 0) and, based on quick testing, that actually seems to help but why exactly does it help? If privacy settings already have a setting to block autoplay, why does that particular configuration change make it work as it is supposed to work? Running Floorp 12.5.0 (Firefox offshoot) on Win11, if that helps.