I can't parse this sentence at all -- mostly, I think, because "true for them" doesn't seem to mean anything. Can you clarify? Also, what's the problem you're talking about?
This is the contradiction I refer to. You can't have truth-entirely-relative-to-a-person and even one person claiming that he believes his truth applies to all. There are options, but they all involve some other sort of absolute axiom that force you to judge some people's beliefs as false as a consequence. Postmodernists try to have it both ways, deflecting all moral criticism with "it's all relative" while still fairly freely wielding absolutes with abandon.
(Incidentally, even if you can't find an academic who actually wrote this down somewhere in some pithy form that could be cited, I have dealt with plenty of people in person at university who did this repeatedly and, to my mind, blatantly. I know I'm glossing over the belief system a bit since this is a comment on the web and not a dissertation, but I know people with this belief system exist. I've personally encountered them.)
You reference a "problem"; were you actually referring to astine's "This is a problem to say the least."? I can't answer to that, those aren't my words.