Yes, Guy, I think you are missing something here. The title of the article says it all:
"JSP.5.1 Uses for XML Syntax for JSP Pages"
...
"A JSP page in either syntax can include via a directive a JSP page in either syntax. It is not valid, however, to intermix standard JSP syntax and XML syntax inside the same source file."
It simply means that it is not valid to write both type of 'syntaxes' for a tag that has both the versions available, e.g.
<%= new java.util.Date()%> // std. JSP Syntax
<jsp:expression>new java.util.Date()</jsp:expression> // invalid if used with above.
This is in cases where standard JSP syntax and XML equivalent are both available and as such does not apply to your example where <jsp:useBean/> is the only available, or standard syntax!
Well this is what I gathered from the specs. I might be wrong though!