Why do Nicolaitan Christians engage Atheists into conversations when Scriptures plainly teach to avoid such people?

Not sure, atheists sure are not interested in these conversations.

Here Paul is referring to Christians that cause division from within, not unbelievers. It was a common theme in his letters. We always need to engage atheists that their souls might be saved by the wonderful grace of Christ. If we all who are believers share the gospel more, the more will be saved. "Nicolaitan" Christians are not true believers, and therefore should not even be called Christian, but should be ministered to with truth, that they too might be convicted, and that they might in due course be able to break the shackles of Satan and become born again.

Engaging atheists does not make you Nicolaitan. We are to be in the world but not of the world. How could they hear the gospel message if we always avoided them.

the nicolatian were satanic, NOT "christian". so you already have a false belief. and no matter what, people should not dispute about scriptures to other christians, but as paul did: contend for the faith, and contend against the church of satan

Technically Christianity has a history of mass converting people. I suppose the excuse is how people with power interpret the message.