That is correct. A work-around is available to make an XTS (Phase I only obviously) or APX scan talkgroups without affiliating with the system if done correctly, and if you have access to the required system key. But there are downsides to this (including the inability to "sit" on a particular channel without programming it as a scan list by itself, a limited delay because you are scanning from a conventional channel, inability to have a priority scan list, and others). The BK Radios, on the other hand, were designed to permit true RX Only programming and monitoring without needing a system key and without loss of typical receive and scan features. Their software is also more freely purchased by anyone with a wallet, rather than having to jump through any additional hoops.
I have a BK KNG-P800, a Unication G5, and a SDS100. I use them all for different things, and they are each better and worse than the other at certain things. Suffice to say I personally prefer the KNG-P800 the most out of all 3, and use it the most when doing routine monitoring and scanning of my local systems. The G5 is helpful for its small size at times, and its better digital decode than the SDS100. The SDS100 is helpful for its Discovery mode, its ability to scan multiple systems simultaneously, it's ease of programming with RR data when traveling, and some other features. But the KNG-P800 radio is the one I'd keep if I had to give up two of the three. It works and sounds better than the other two in its intended application. I like the SDS100. It is better at P25 than any other consumer grade scanner I've used. But that isn't saying much. And I suspect those who think it is close to perfect for any P25 system just haven't spent much (or any) time with a KNG radio programmed for true RX only mode and priority scan.
