Leaving aside the fact that its not an infallible statement, the context of that is Protestants who are entering into mixed marriages with Catholics. Its at least highly, HIGHLY unlikely that someone who's married to a Catholic could be invincibly ignorant. Maybe even impossible.
Furthermore, since its not an infallible definition, its possible there's some wiggle room. Its not obvious to me whether he means *every single* Protestant is damned, without exception, or if he's making a more general statement with the intent of stomping out indifferentism. Because that seems to be the context, from what I recall. Catholics marrying Protestants casually, as if it didn't make a difference what religion you're part of.
I'm not convinced there's a straight line between this quote and Feeney. I also find it difficult to believe Lefebvre was simply ignorant of that quote.