The fact that the MCP and “5 min limit” are derived from the graph to allow the same power into the transmission at different altitudes 205hp cont/225hp for TO (from distant memory - R44 R1) would indicate it’s mainly a gearbox saving limitation.
Once you reach an altitude where the engine can only produce 205hp (full throttle height) you are allowed to run the engine wide open all day long with no TBO penalty.