Is there any reason to be confident that was MM's full power and not just a small test to see if the system worked? Or that even if it was his full power at the time he hasn't invested in more hardware?
No, there's no way to be confident given that MM scaled up and then stopped once they reached 50% of the total network power.
But scaling up to 280 times their earlier rate is not an easy thing to do, and is not as simple as buying 280 times more hardware: MM would require additional floorspace likely in a hosted location, HVAC, more power outlets, more breakers, bigger power drop, etc. If each machine/device gets 1 GH, they'd need 140,000 machines/devices. There might not even be enough floorspace in a local hosted location, and they'd have to construct their own facility to house the operation, a multi-year project in and of itself.
Also, there's installation time: it took MM two weeks to scale up to 500 GH, and at that installation rate, it would take 10 years to install 140 TH. Clearly it would need to be a whole team to work on such a project, perhaps two: one larger one for install, and another for upkeep.
MM's scale and timing was just about right for an unused or underused render farm between projects. 500 GH is a couple of thousand machines, and three weeks of scaling then suddenly ending sounds about right from a business perspective, too.
And if you are still worried, support your local FPGA miner! It's the only way to attempt to scale up and compete with this or any other future Mystery Miner.