It was launched in international waters and took a course away from the CONUS. Assuming we have a defense against it and assuming it was spotted immediately then it follows that we assessed it as a non-threat and did nothing instead of revealing our capabilities.
I lean toward the speculation that it was ours but if it was, for instance, China's and they were probing our responses they learned little for certain. They might know from radio chatter whether we were on it or asleep at the wheel but it may be very difficult for them to ascertain why we didn't fire at it.
They can only guess that we have no capability to, are too slow on the trigger, have no will to respond, have constipation in the command structure or, as I laid out above, we had it dead in our sights and correctly called it no threat within seconds. I'd love to believe that last one but...
>> It was launched in international waters
I haven’t been studying this to any great extent, but between the lighting and visually decreasing width of the ‘trail’, the trajectory appears somewhat perpendicular to both the surface of the Earth and the visible cloud cover.