That would be similar to my guess. On an OFT system, you can only have one transmission in progress at a time. There is no "other frequency" for the scanner to review, since it can only receive one signal at a time. Unlike a true P25 system, where there is a control channel that can be checked for a Priority TGID in use, there is none for an OFT system.If you were to compare the Options for a 1 Frequency P25 system with a "regular" P25 trunk system you will see that there is no Priority ID Scan option. Since there is no Priority ID Scan, there isn't a reason to set priority channels.
Why there is no option to do Priority talkgroup scanning, I am unable to explain other than hazarding a guess is that there is no reason for a priority due to the nature of the system.