I think I misstated or misunderstood the restriction. The restriction I'm
referring to is:

"There are restrictions on the use of the xsl:result-document
<http://www.w3.org/TR/xslt20/#element-result-document> instruction,
designed to ensure that the results are fully interoperable even when
processors optimize the sequence in which instructions are evaluated.
Informally, the restriction is that the xsl:result-document
<http://www.w3.org/TR/xslt20/#element-result-document> instruction can
only be used while writing a final result tree, not while writing to a
temporary tree or a sequence."