I'd say it's rude in general to tell someone they won't find something interesting, and especially so when it's done as a way of disowning your own preference not to talk about it.
For example, when people ask what I do, I could say I work at an ISP, or that I'm a systems engineer (my title), or that I'm a system administrator (some of what I do), or that I'm a software developer (the rest of what I do), or any number of other things. Depending on how interested in it I think they will be, or how interested in explaining it I am, I might respond that it's likely not that interesting.
If the person asking actually wants to pursue it further, the polite thing to do would be to say "oh, I find it interesting, if you're willing to talk about it". If they responded "How do you know what's interesting to me?" I would take that as somewhat aggressive, and definitely wouldn't be interested in explaining further, depending on how I perceived their disposition.
Perhaps it's a cultural miscommunication.
If they get aggressive and you then decide that you don't want to explain, that's fine. But that's not what happened here.
Eh, I don't think you can say that definitively. I took it as WaxProlix not wanting to/being unwilling to talk about it because it's boring, and as they found it boring, they thought other people would likely (the word they used, which I think people are ignoring) find it boring as well.
They apologized to the person they said it to, and it all seems settled from my perspective.