Looking at these two options as provided - winnowing and focusing:
The problem with winnowing is the risk of accidentally winnowing out the item you're actually looking for - i.e. throwing the wheat out with the chaff. You have to know what is safe to remove, which means you need to have an idea of what is most likely required to keep, and if that's the case, then isn't it annoying to start with all selected because you have to deselect all the others to hone in.
Focusing is more systematic, and does not suffer from the same problems as winnowing - you might skip past the selection with the desired item in, but that's slightly different.
Focusing allows you to look at what you think is the most likely category with a single click and if not found, to switch to another category with one more click.
But winnowing makes it less easy (more clicks) to select a single category if the user has a 'gut feeling' as to which is the right category. Subsequent failure to find an item in that category then requires deselection, and then selection of another (more clicks)
So, focusing initially seems the best bet.
However - there also side-benefits to winnowing, in that for a new user, it's a better way of presenting greater amounts of information - it allows exploration and discovery. Focussng is better for users who already have some knowledge of the content and can make those informed decisions that ease the process. A new user probably has no reason to focus on any one option over another. Winnowing increases the chance that the next time the process is undertaken, the user has a little 'previously learned information'.
Consider whether new users will be quickly getting to grips with the content, or whether this could be a slow process.
So Winnowing seems better for new users - focusing is better for experienced users.?
But there may be additional factors which make either of the processes more difficult. For example in the case of a larger number of categories, winnowing becomes increasingly harder, because, the reasons to discard one category have to be more firmly held; the confidence that the desired result is in the remaining categories is harder to determine (as opposed to being thrown out with the chaff); and the ease of back-tracking is reduced. In all likelihood, failure to find a desired item will result in reselecting all categories and starting again - a point of frustration.
In this case, the focusing option (already identified as the more systematic approach) should prevail, especially if the presentation of content is made easily traversable. For example, compare ease of scrolling up and down a long section of content versus paginated displays. Being more systematic, as the user progresses from one category to the next, the confidence that the desired result is within the remaining categories is increasingly greater with each progression.
So As complexity increases, systematic focusing prevails.
Consider each variable and implement accordingly.