It's not in a place that would be dangerous, so I wouldn't worry too much about it.
That's the bit I was mostly worried about.
I've read a few things about cracks being stopped by drilling, but wasn't sure if the position of the crack meant the damage was already done, especially as it kinda looks like the top bit of metal is starting to overlap the bottom bit.
It might also explain the chewed-up area around the seat bolt. Presumably a previous owner had a seat post that was 26mm, and kept tightening, and tightening, and tightening..