The only way "icing would be nearly unavoidable" is if you had supercooled liquid water droplets sitting around and waiting for you. In many situations they are either already frozen, or not cold enough.
The OP's 23F OAT (-5C) is certainly in the "danger zone", but the droplets encountered could have already been frozen when the aircraft met them, at which point they'd bounce right off.
As I mentioned above, in my experience very often the droplets you meet are frozen, so there is no icing, therefore this case wouldn't be at all surprising.
It's also possible the droplets were liquid but very small, which would have required a very long time to get an appreciable buildup.