This is definitely not always true. For quite a few years now, I have had nothing but nightmares trying to manage TATL flights on LH-group airlines with an 016 ticket. The only reason I kept booking with UA was because I was able to get a markedly cheaper airfare for the same exact flights on UA.com, which seems to be less of an issue lately. I have had very little success accessing the online seat selection after logging into the LH website or app with the PNR. I usually have to call or DM the airline to select or change seats. I have been able to do it online maybe 10% of the times I have tried in the last 5 years. This is true for LH/OS/LX flights booked via UA. The correct PNR will allow you to view the flight info, but many of the options, including seat selection, are disabled. There are also sometimes where I need to enter the UA PNR instead of the LH PNR, which makes no sense. The bottom line is that there are a lot of needless hassles that arise accessing a 016 flight on LH-group websites/apps.