Look at it from the other perspective: If the Coyotes owner refused to retain on Dumba and Zucker, it probably eliminated all the cap strapped teams from being in the race.
Dumba’s $1.35m Signing Bonus is reported to be due after the trade deadline. Tampa will end up paying around 48% of Dumba’s $3.9m compensation this season. $1.35m signing bonus + ~$540k salary.
Arizona would need at least a 3rd round pick in return simply to justify 50% retention of $900k+ on Dumba’s remaining ~$1.9m compensation this season. But Dumba isn’t worth a 3rd, even on 50% retention.
The trade without retention for only a 5th makes a lot more sense for the Coyotes once you factor in Tampa is paying the signing bonus.
Expanding further: if Dumba has been worth a 2nd with 50% retention at the trade deadline—as Arizona probably planned when they signed him last summer—I’d expect the Coyotes would have retained that 50%. Alternatively, if Arizona had paid Dumba’s $1.35m signing bonus then they would have happily retained 50% (~$270k cash) for a 3rd or 4th at the trade deadline.