Cotten was running a Bitcoin exchange. There were no promises of returns on deposits and people were under the impression they were buying or selling their Bitcoin to others on the exchange. Cotten stole people's Bitcoin and money. What happened is a fraud and a theft but by no means was QuadrigaCX a Ponzi scheme. Cotten was an embezzler.
The Ontario SEC needs to up their investigations game a bit.
Technically speaking he wasn't running the classic ponzi scheme but according OSC it seems he has used the "technique" of using new funds deposited to pay older users.
So probably he has just avoid to pay immediately withdraw requested and waiting for new/fresh deposits on the exchange.
Otherwise if your exchange is able to offer better condition respect the market, also a better % or a fee advantage, despite he is not offering explicit a "fixed returns" it seems like a ponzi scheme since better condition = will allow more people to deposit here.