Is it true that non-denominational churches are really Southern Baptist?

Yes.
Non-denominational churches define themselves from what they are against…traditional religion. That’s fine. No problem there. If you don’t like tradional religion that is your prerogitive.
But then, what is it that defines who you are?  What do you believe in?
Non-denominational churches (for the most part) are’free will’ ‘decision theology’ biblical fundamentalists. Their theology is Southern Baptist.
So [...]