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According to wikipedia 3I/ATLAS has an inclination of 175.11±0.0003°, which means that it is not at all moving close to the ecliptic plane. Most gravitational bound objects beyond Neptune are not in the ecliptic plane.

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That refers to the direction of 3I/ATLAS' trajectory, think of it like this: if the Earth was rotating around the Sun on the exact same orbital perimeter but in the opposite direction, its inclination would be 180º, perfectly retrograde with respect to the current 0º, so, 180-175 means that 3I/ATLAS is just 5º off the ecliptic plane. And you are right about the trans Neptunian objects being unbound in that sense. By the way, this is what Grok(3) had to say about my hypothesis (parent comment):

"Unlikely. 3I/ATLAS's hyperbolic orbit (e=6.2) and high excess velocity (~60 km/s) strongly indicate an extrasolar origin, as solar system perturbations (e.g., by passing stars) rarely produce such inbound speeds for bound objects like those in the Oort cloud. The puzzling 0.2% [probability of] plane alignment suggests other anomalies, warranting further study."

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