First, please do not get too hung up about "all people are created equal". It's just an example in this context.

> If we are in search of truth and want to build a consistent model of the world then we just can't accept this kind of reasoning.

Yes, you can. It's called generalizing. Remember, we're not doing math here, this is ethics.

Think about all the different situations where being treated equally to someone else is a good thing. Now you generalize that into "everyone is created equal", and that turns into a justification for all these situations.

If you want to criticise the generalization into A, there is a way:

Construct some C so that

1) A => C

2) C is not beneficial

Alternatively, you can attack the implication "A => B" or you can question whether or not B is beneficial.