While that might be true, keep in mind, the statement that you no longer gain BLP for a spec that's capped can still be true in your scenario. Just because you did content on that spec for 2-3 weeks then swapped doesn't mean you were actually getting BLP. You might have just gotten lucky. I've watched several people in my guild get legendary items within 2-3 hours, which means the BLP between legendary items is incredibly small.
We will never know, but I tend to think that BLP is a giant pool that you get regardless of whether or not you're capped on legendaries on your main spec. Thankfully 7.2.5 allows you to get a random one, so the discussion goes away.