Bitcoins are legitimate property not because (as in the case of "IP") a right to control others' property is asserted, but because an individual has sole access to the private key (code) that controls access to the Bitcoin(s).
Bitcoin ownership is more like ownership of an unshared idea: a design or invention that only one person knows, for instance. (If another person came up with either independently without harm—highly unlikely in the Bitcoin case, but theoretically possible—then they would have as much right to it as the other "owner".) It is a person's private key, note, that is equivalent to the "unshared idea", not the Bitcoin identifier itself, which is more like a label for an invention (the term "Robins-design warp drive" vs. the plans for a warp drive).
And of course, if a person shares the idea (manifests it physically and releases the manifestation, e.g., a paper or electronic book) or private key, they can no longer expect to be the sole owner of either (modulo Rothbardian contracts).