re. aircraft manufacturer's engine operating *recommendations* (not limits): Nothing in the POH says that the engine is guaranteed to make TBO if the recomendations are followed. Similarly, no certification regulation specifies that TBO must be guaranteed. Those recommendations are purely to make the POH performance numbers.
In a similar vein, think of the old turbo F1 qualifying engines: 1200HP from a forced induction 1.5L engine - and it was toast after one round of qualifying - but, jeez, the performance was fabulous. Great for advertising capability. Meanwhile, the same engine producing only 800HP would (usually) last an entire race but performance was rather lower. Much like POH engine operation to achieve POH performance figures, vs LOP ops to maximise engine life.