Steam machine uses about 200W. Can you even buy any 240W pd power bricks. Quick search on amazon shows that all that advertise 240w can only output 140w max on single cable
That said I would not trust this as a PSU for a computer which uses almost all the available power output and does not have it's own internal battery. Most USB-C chargers are not designed to run at 100% capacity for extended periods and could not handle sudden spikes over the rated capacity. Even for desktop PSUs you always want to get something that has a good headroom over the actual power draw for stability.
That said I would not trust this as a PSU for a computer which uses almost all the available power output and does not have it's own internal battery. Most USB-C chargers are not designed to run at 100% capacity for extended periods and could not handle sudden spikes over the rated capacity. Even for desktop PSUs you always want to get something that has a good headroom over the actual power draw for stability.