Why would he include a hypothetical spoiler candidate in his own internal?
Politics is about being covered under all eventualities.
Presumably an internal poll is performed for the benefit of the candidate. While it would be smart for said candidate to include a hypothetical three-way race in said internal poll to evaluate the potential effect a spoiler would have, it is idiotic to actually release those numbers rather than head-to-head numbers, as the latter numbers will almost certainly look better for fundraising purposes.
EDIT: I thought these were an internal for the general election, not the primary. Including Miller suddenly makes sense, although I don't see how Treadwell wins the primary if he can't lead his own internals.