Originally Posted by
cyccommute
That's a hell of a fender brace! Look at the buckling of the downtube again. It is very deep (roughly 10% of the tube diameter). The tire would have mitigated damage from something as light as a fender bracket.
The fender brace didn't have to be strong, it was the meat in a sandwich. It was trapped when the tire was driven into the tube. If this bike didn't have fenders, he would have ended up with a broader shallower dent, or maybe no dent at all. However there wasn't enough force to bend the entire tube upward or change the geometry of the frame (as appears in the photo). The OP can confirm that with a straightedge, or by checking the front wheel position with a new fork.
Obviously none of us were there, so anything we say is opinion. It's also opinion as far as predicting the outcome if the OP chooses to ride the bike. My opinion is based on what I see in the photo, and 45+ years working on bikes.