I also think it looks/feels “funny” having a source tag with src attribute…
but I disagree.
You can understand the syntax by example, without knowing the spec.
When a single “video” has multiple “sources”, obviously each source must contain more information than just a URL.
The URL must be in an attribute value (everything else would be inconsistent and introduce whitespace problems).
“href” would be the only meaningful alternative, but, as the author points out, “src” is what’s been in use for similar content in HTML.