The horizontal axis on that graph is logarithmic. It doesn't go 1, 2, 3, 4... it goes 10, 100, 1000, 100000,...

Yeah, if you look real close at the steel line (the blue one) on the graph eventually becomes a straight line, but not perfectly horizontal, meaning that at a bazillion-bazillion-bazillian cycles even low stressed steel would break.

Up through the 1960's and maybe 1970's, cylinder heads on BMW six-cylinder engines would eventually crack due to fatigue from thermal stress, even if they never overheated. The joke was when shopping for a used six-cylinder BMW, if they didn't have a receipt for a new head, deduct the cost of one from the price.

When they rebuild piston aircraft engines, they don't rework the cylinder heads. They just throw them in the recycling bin and install new ones.

A lot of chapters in engineering text books end with a paragraph saying the information in the chapter is mostly, practically, but not entirely true.