The idea of multicellular life on Ceres is implausible for the reasons you mentioned, but there could be something like archaebacteria or other extremophiles with very slow metabolisms and which don't reproduce very often. As it's a brine ocean, the water could be chemically "hot" enough that a form of life is sustainable in a bio-geological cycle.

Here's an interesting article exploring the lowest possible temperature for life on earth (which is largely focused on vitrification blocking cellular metabolism but has both theoretical and empirical insights): https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3686811/pdf/pon...

If that's the case why would it be an ocean at all, rather than a block of ice?

The article said it was a brine ocean, so salinity or some other solute - even at Earth gravity and air pressure, highly saline water can have a freezing point as low as -20C.

The maximum surface temperature on Ceres is -118C. How would you get to -20C without geological heating?

Pressure? Water unlike most materials expands when it freezes.

When you apply pressure to ice, does it melt or freeze?

Google says, https://van.physics.illinois.edu/qa/listing.php?id=1593