I'd like to think the plate did escape the Earth, some of the upper estimates were about 42 miles per second, (which ironically is about the maximum speed the Voyagers ever attained during the Jupiter/Saturn grav slingshots) but the shape was hardly aerodynamic, and there's no accounting for losses due to tumbling, or compressive/frictional heating.
Meteors that are at least roughly spherical, and massed more than the plate, and coming in at that speed tend to not reach the ground either.
Also, that was the plate's maximum velocity at the detonation site, it would have been doing nothing but slowing after that, from the atmosphere, then the Earth's gravity, then the Sun's.
Then, even if we simplify the launch direction, and assume it was largely straight up at the zenith, what time of day was the test? Anything other than a night shot well after midnight and significantly before noon (Say between the hours of 3am and 9am...) would be robbed significantly of velocity by having a nominal retrograde launch against Earth's orbit about the sun.
And if it's launch were orbit neutral it would have largely had mostly a straight velocity out from the sun, and just fallen back on a long hyperbole.
I say 1/1000 odds it left the atmosphere. And 1/100 odds it's left the Solar System.
