By definition YAML isn’t markdown, but many programs accept it for the utility of the Front Matter conventions. The — fences are a simple visual cue - most people prefer it to counting backticks.
For some uses I prefer to have Metadata parsed as part of the main text, but usually I want it excluded and not rendered. And it’s nice when it works the same in other programs and the YAML solution is the only way that does that.
There’s huge inconsistency in implementations of comments in markdown. Some work in Obsidian, others don’t, but none are a commonly accepted way of indicating a Metadata block.
And some people prefer to have their YAML block at the end rather than the beginning, but still refer to it as Front Matter.
Personally, I’d most like the developers to decide soon. I’d rather implement a method than not, but it’s off-putting to be aware it might have to be changed.