Is a loose psu plugged into a three prong outlet but off a good thing to clip to an esd mat aligator clip?
If the &quot;old PSU&quot; is a desktop PC PSU then it's case is almost certainly connected to ground. So yes the case of an old desktop PC PSU could be used as a grounding point. Laptop PSUs on the other hand generally do not have anything accessible connected to mains ground.
For ESD protection you DO NOT want a low impedance connection to ground, for two reasons. Firstly a low impedance connection between your body and ground singificantly increases the risk from electric shocks, secondly the whole point of ESD grounding is to avoid sudden discharges, a low impedance ground works against that goal.
Instead you want a much higher resistance (the exact value is far from critical), this lets any static charge discharge slowly and limits the current if you somehow manage to shock yourself. ESD grounding equipment often contain resistances of around 1 Megohm and it's not a problem if several end up in series. However there is some ESD gear out there that is low resistance, potentially ok as part of a larger system with the resistor in a different component but a big problem if it's used on its own.
Your kit quotes a resistance of 105Ω in it's grounding cord, it's not clear if there is also a resistance in the earth strap. 105Ω ohms seems lower than most I have seen and I think it may be a typo, because the photo of the cord says 1M. 105Ω is a bit on the low side  for current limiting (it would allow 1.2mA at 120V, which while unlikely to be deadly but is still higher than I would like).