No, the “Main Street open an hour before park opening” thing only applies to 9AM park openings.
For an 8AM opening, the front gates will open 7:45ish. “Ish” is a key word as exact procedures can change day to day. The welcome show will be on the castle stage at 7:55AM, after which they’ll “drop the ropes” at the various spokes around the hub, and the park will be open.