You are safe using == with floats if they have an exact representation (for instance are an integer), but its a bad habit to get into...
Back to the problem - those chips have no hysteresis on their alarm conditions so will flicker for a bit on temperature changing past the threshold slowly. I agree to ignore all that alarm stuff and test the temperature(s) yourself - you can add hysteresis and delay as you see fit and your code will be smaller as you won't drag in all the DallasTemperature library's alarm code.