My take on it is - the ships from Minefield
don't have cloaking technology, just the same kind of holo/stealth technology seen later in Babel One
. Another thought following on from that theory is, that Bird of Prey design doesn't even need to exist beyond the drawing board, or a failed prototype. The Romulans can deceive neighbouring races with an armada of drone ships, all projecting false images of a far more advanced design.