In a situation where communication between the radio and Naza is lost - ie a fly-away - DJI are assuming that there will be no input change on the control channels, which is probably a fair assumption if the loss is down to external interference.
However, 20 seconds is a long, long time when something goes AWOL, plenty of time to go up very high, into something or well beyond clear line of sight.
It might be good to see an advanced option pane in the naza/phantom assistant which allows (advanced) users to change that delay time to something they feel more comfortable with.
Personally, I'd be happier with a 3-5 second failsafe on input loss (freeze?).