lacks the most important information, namely the <divisions> element. Since all elements in the shown example are half notes with <time-modification> elements it is impossible to deduce how to properly modify <duration> for the tremolo notes.
An alternative to showing <divisions> is to modify the example, displaying a tremolo and a non-tremolo part.
The double-stem tremolo example given in
https://www.w3.org/2021/06/musicxml40/musicxml-reference/examples/tremolo-element-double/
lacks the most important information, namely the
<divisions>element. Since all elements in the shown example are half notes with<time-modification>elements it is impossible to deduce how to properly modify<duration>for the tremolo notes.An alternative to showing
<divisions>is to modify the example, displaying a tremolo and a non-tremolo part.