Most of the views on this seem to be from the SF/Fantasy viewpoint and I think it will be even harder to define ‘indie’ in the historical arena.
Long established companies such as Irregular, Baccus, Adler and Peter Pig (and there are lots more in the same vein) are still small, usually identified with one person (or a small group) and have a good track record of innovation. Most historical wargamers would think of these as mainstream because they produce a large range of figures. You just don’t have the tie-ins to rule systems as commonly in historical wargaming – it exists but only in a relatively small part of the genre.
For me, it isn’t really a particularly useful distinction. I usually know roughly what I want and in finding it I’ll scan the offerings of any supplier, whatever their size, genre or background. I can see why it might be a useful label for others in the more ‘imaginative’ areas of gaming though.