In other words, Christadelphians reject the doctrine that the Kingdom of God is 'the church.'
As with many of the DTBR this statement stands alone without any explanation or clarification and has led to enormous confusion. Note that it does not say that Christadelphians reject the notion that the church is the Kingdom of God, but rather that the Kingdom of God is 'the church.' That might sound like pedantry, but it's an important distinction which many Christadelphians have noted.
In other words, the statement of faith rejects the view that any and all references to the Kingdom of God in Scripture are referring to the church; but that is not the same as saying that the church is the Kingdom of God. It's like the difference between saying "the ocean is made of water" and "this is water, therefore it's part of the ocean". The church can be part of the Kingdom of God, but the Kingdom of God does not comprise only the church.
Confusing? It sure is. That's one reason why many Christadelphian ecclesias these days don't include the DTBR in their statement of faith - they raise more questions than they answer.
However, one of the casualties of this awkward way of defining "the truth" both positively and negatively (i.e. "Truth to be received" and "Doctrines to be rejected") is that it has led to confusion on several of the DTBR. They say what Christadelphians don't believe without necessarily saying what they do believe about the matter. Some people mistakenly read clause 12 as meaning that the church is not part of the Kingdom of God, and therefore react against any suggestion that the Kingdom has a present reality which is manifested in the church.
But Jesus came to establish the Kingdom, and to build His church. He called people to live in community as the people of God, now and in the future. The kingdom is "now" as well as "not yet" - it has a present reality, and a future consummation.
The church, the assembly of (new) Israel, the holy nation, the people of God, exists today as the first stage of the Kingdom of God.
The teaching of Jesus about this is so clear that the only reason I can think of why so many Christadelphians have missed this important truth is that they have been conditioned by this confusing and awkward negative clause in their statement of faith which steers them away from thinking in any terms which might identify the church with the Kingdom.
The church is made up of Kingdom-people, living Kingdom-dynamics. They live not only in expectancy of the Age to Come, but they are empowered, enabled and energised by the powers of the coming age to live a Kingdom-life today.