It seems the spec indeed allows this.
Furthermore, Saxon-JS 2 doesn't complain:

Saxon 10.1 does:

The closest test in the test suite I could dig up is
https://github.com/w3c/xslt30-test/blob/master/tests/attr/match/match-082a.xs
l,
it solely uses a parenthesized union pattern

    <!--
        Purpose: Parentheses in patterns become legal in XSLT 3.0. They
have the same priority.
    -->

    <xsl:template match="(doc|cod)">
        <wrong />
    </xsl:template>

    <xsl:template match="doc">
        <ok />
    </xsl:template>