Having the behavior cleared I wonder how useful it is in practical terms. A class type hint guarantees me I can do a specific call to methods defined in the class/interface. The proposed type hint tells me I can call it in some way. It won't ensure that the signature is compatible with what I expect.

function foo(callable $cb) { $cb(); } foo("strpos"); // This one in fact is illegal but won't be prevented

But maybe this doesn't matter as type hints purely serve documentation (as E_RECOVERABLE are useless unless we make them Exceptions ...) while even for documentation purpose more information is needed.