I needed something really cold, so added a heap of salt to fresh water. Freezing point was lowered significantly. When I measured the temp of my brew it was -17°C.

Ice can indeed get colder than it's freezing point .....way colder . I can't see why it couldn't get to close to absolute zero. But water cannot get colder than it's freezing point ( and that can vary a bit like the boiling point) after that its solid ice .
Similarly water cannot be heated above it's boiling point (100⁰c at sea level ....I think we have all been at sea level 😁), but we can mess with that as water boiling point is dependant on ambient pressure. Increase pressure and the boiling point goes up ( which is why we run pressurised cooling systems on our cars etc) boil a pot of water on top of Everest and it won't get near 100⁰c because atmospheric pressure is lower.
This manipulating of boiling points of liquids is how we get refrigerators to work. Some refrigerants boil at -40⁰c.
Anyhow 😁as far as practical for us fishers the freezing point of ice ,be it salt or fresh , probly makes feck all difference.
I have a 60l Coleman chilly bin and two bags of fresh ice ,days fishing ,left the Esky full of slurry on veranda and the last cube melted damn near a week later, thusly I have not bothered to get a lectric chilly bin , preferring to spend that money on various bottled (and canned ) liquids to put in said chilly bin.
Here is a hot tip .... don't let you bestie break up ice cubes with the boat knife.......stabbed a **** load of holes in my chilly bin liner🤬🤯

Here's a copy of something I posted 10 years ago with a "recipe" on making salt ice at home to hit specific temperatures....."Here's some info from an Aussie chilly bin maker's website (www.xtracool.com.au) on making salt ice to keep your bin cool.  I haven't tried it yet but seems to make sense and they must have even greater challenges over there keeping their catch cool than us: