Often those who can't help but think in denominations use this objection to say that non-denominationalism is, in effect, another denomination.
The misconception is the failure to realize that non-denominationalism is not an organization but a belief and a conviction like Trinitarianism, the belief and conviction, derived from the Bible that there is one God who exists in three blessed persons.
A denomination is an organization that excludes some Christians from their fellowship if they don't accept certain distinctive doctrines that the organization holds dear.
Non-denominationalism, on the other hand, is a belief and a conviction that embraces and recognizes all those who trust in Christ as part of the one body. To avoid any semblance of divisiveness, they don't do anything and say anything that will go against the one body principle.