The 26 km/s is a hyperbolic excess velocity. Not so much its current speed (though this is quite close), as how fast it would be going in an idealized case where it can get arbitrarily far from the Sun and we can ignore the rest of the galaxy, etc.
Its encounter with the Sun changed the direction of it's motion, but not its speed. It's path was bent 66° by its trip through the Sun's gravity well, but for all practical purposes – treating the Sun and Oumuamua as a simplified two body system – Oumuamua left the solar system at the same speed it entered it.
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UmbralRaptor t1_j5gm1e8 wrote
There's two things going on here:
escape velocity is related to distance. eg: Solar escape is ~42 km/s at Earth's distance, but some 600 km/s if you start at the photosphere. see eg: https://en.wikipedia.org/wiki/Voyager_2#/media/File:Voyager_2_velocity_vs_distance_from_sun.svg for how it falls off.
The 26 km/s is a hyperbolic excess velocity. Not so much its current speed (though this is quite close), as how fast it would be going in an idealized case where it can get arbitrarily far from the Sun and we can ignore the rest of the galaxy, etc.