why would two-bit mathews think johnny, dally and pony were heroes all along, before they saved the kids and were published in the newspaper?

"What I like is the 'turn bit," Two-Bit said, "Y'all were heroes from the beginning. You just didn't 'turn' all of a sudden"

As you can see from the above quote, Two-Bit believes in his friends. He knew that his friends were brave and caring from the start, whereas everyone else judged them by the way they looked. Neither of the boys showed any hesitation in coming to the aid of the children trapped inside the church. What it comes down to is that some people can see past the way people look or dress and see the people inside. Two-Bit saw his friends for what they were.

But it still doesn't explain why Two-Bit is a hero, its just saying that his friends are heros.