existence of "firstNo" at the start of non-reorderable sequences not checked

the xliff file

<xliff version='2.0'
xmlns='urn:oasis:names:tc:xliff:document:2.0' srcLang="en" trgLang="fr">
<file id="f1">
<unit id="u1">
<segment>
<source><pc id="1" canCopy="no" canDelete="no"
canReorder="no"><ph id="2" canCopy="no" canDelete="no"/></pc></source>
</segment>
</unit>
</file>
</xliff>

is recognized as valid by xliff_core_2.1.sch (rev. 807), event though
in
http://docs.oasis-open.org/xliff/xliff-core/v2.1/csprd03/xliff-core-v2.1-csprd03.html#canreorder
says:
"firstNo when the code is the first element of a sequence that cannot
be re-ordered, no when it is another element of such a sequence."

Could you please have a look?