why would two-Bit think Johnny, Dally and Ponyboy were heroes all along, before they saved the kids?

Two bit already knew the character of his friends. He had seen Dally's passion, Pony's intellectual softness, and Johnny's empathy. He knew this long before the fire,

"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" (107).