Have you read Good Subjective, Bad Subjective? It makes it pretty clear that there is a continuum of subjectivity that begins immediately after "Does this code compile?" (RPG.SE equivalent: "Is there a RAW or other factual answer?") and proceeds onward from there by degree. It's therefore a false dichotomy to ask if a question "is a subjective question." Most of our questions are at least partially subjective. The entire article was inspired by the moms4moms site which was by its nature mostly subjective as compared to the coding questions on SO, and that guidance was for their entire site's operation!
GS/BS is a not just guide "to voting," many of its rules are in all sites' help centers for a reason. See https://rpg.stackexchange.com/help/dont-ask. That's not custom for RPG.SE (we can't even edit it), it's all about not asking "bad subjective" questions as overall SE guidance.
The Back It Up! principle is therefore relevant to whatever portion of a question is subjective.
As mentioned in How do we ask and answer subjective questions? (which is where this discussion probably belongs) I know people think they know everything and can easily extrapolate. And sure, relevant experience doesn't have to be totally exact. But it's just like the value of playtest info over the value of armchair theorycrafter info - it's just plain better.
For "Running Forgotten Realms in Pathfinder" - that's right, if you can't explain that you've done that, or seen it done, or even directly analogous experience ("Well, I converted Eberron to PF fine and it's just about the same process"), or can otherwise back it up (citing others' experience, facts...) then it's not a valid answer. It's certainly not that you're a bad person or your theories are necessarily wrong - it's just that there's no reason to believe they're right besides appeal to your authority. And when everyone does it, to quote the meta.SE blog post, "you are left with an experience that looks more like the magazine rack at a grocery store than a book shelf at Harvard."
Everyone on the Internet thinks they know things. Here, we specifically want expertise. Experts have actually done things. It's pretty simple and direct. If you can't Back It Up! then it's not a good answer.
In this case, a question about running the Realms in Pathfinder, one might legitimately expect expertise, personal or cited, in that actual topic instead of "converting from 3.5e to Pathfinder's easy." The OP cited a conversion guide. "I'm sure it'll be fine, because game system conversion is easy" is no better answer than "Oh you can use FATE for that because it's good for anything" or any other such. In this case the answer's bad because it's a mix of lacking expertise and also not answering the real question (game system conversion from 3.5e to PF is an existing question with answers - how does it bear specifically on the Realms?).
See How do we ask and answer subjective questions? for more on site policy on subjective questions.