If they have the means to compress air, they can make both hot and cold.
Compressing a gas, plain old air would do, heats it up.
This heat can then be allowed to exit into the environment.
When the compressed air is released, it cools down.
Enough of this cooling, will form water Ice.
It is easier to do if you have access to metals, and mechanization, and electricity.
But none of these is necessary, merely convenient.
A large clay or wooden or glass cylinder, with a tightfitting seal (think oversize bicycle pump) will do well enough.
Compress your air at one point, get rid of that heat, (running water comes to mind), then pipe the compressed air to another location, where it is released while in contact with the water you want to freeze.
You don't need extraordinary pressures for this to work. For example, nice sturdy glassware piping will do fine. Maybe even for the primary compression cylinder.
According to these handy online calculators:
Energy from compression change in Air
Latent heat of Fusion for water
If you compress 1.2m3 of air at 30C down to 1m3, let it cool, then release
you cool the air exiting to -3.1C
And absorb enough heat to freeze 125g of cold water to Ice, or cool 330g of water down from 30C to 0C (but still liquid)
Even a wooden cylinder can take 0.2 bar of pressure without strain.