Magento2 PHPUnit internal method getCollectionMock. Bug or Feature?

When I was writing unit tests for one magento collection I faced with the following strange thing:

Magento allows create mock for collection with it's internal phpunit method:

\Magento\Framework\TestFramework\Unit\Helper\ObjectManager

public function getCollectionMock($className, array $data)
{
    if (!is_subclass_of($className, '\Magento\Framework\Data\Collection')) {
        throw new \InvalidArgumentException(
            $className . ' does not instance of \Magento\Framework\Data\Collection'
        );
    }
    $mock = $this->_testObject->getMock($className, [], [], '', false, false);
    $iterator = new \ArrayIterator($data);
    $mock->expects(
        $this->_testObject->any()
    )->method(
        'getIterator'
    )->will(
        $this->_testObject->returnValue($iterator)
    );
    return $mock;
}

The problem appear when I'm trying to create Mock of \Magento\Framework\Data\Collection.

As you can see magento checks if described class is subclass of \Magento\Framework\Data\Collection. Not instance of!

Magento returns the described collection here:

$product->getMediaGalleryImages();

The question is: is it a bug or a feature?