Circuit simulations ... arise because the op amp isn’t really perfect. A real op amp would not hit the 15V rail exactly. It will get close depending on how much current you draw from the output.

The humble op-amp can be configured ... transistors and the lack of a current mirror in the long-tailed pair, but using those to find fault in a circuit built for learning is beside the point.

Part 5 provides spec summary of the GP/R K2-W vacuum-tube op amp and discusses measurement constraints ... modeled result shows 1.00 nA of input current. Early SPICE simulations based on that ...