Contradiction in xml_set_default_handler() docs

From manual page: https://php.net/function.xml-set-default-handler

It says:

Description
xml_set_default_handler(XMLParser $parser, callable $handler): bool

Ok so $handler is a callable. We all know what a callable is: either a string which is the name of a function, an [$object, 'methodName'] pair, or an anonymous function definition.

But then it says:

handler
handler is a string containing the name of a function that must exist when xml_parse() is called for parser.

Wait, so must it be a string? Or is this just a leftover from an older version when callables didn't exist and it actually had to be a string with a function name?