Not a surprise and not a disappointment either, when it was clear from the start that all results are within measurement error bars, so that there was neither experimental nor theoretical reason to expect to see something.
It also would have literally meant the end of the world in a short period of time. Reactionless rockets with little to no cost or requiring high technical capability would lead to a terrorist shortly speeding up a hunk of lead to near lightspeed and slamming it into a earthside target, and that would be that.
We've had a few thousand nukes, many secured with the password 0000, lying around for a few decades now in a dozen countries, and that hasn't materialized. For some reason, we seem to be much better at imagining ridiculous scenarios for terrorists than they are at implementing them.
Well first that need to get it into orbit and outside of the suns gravity well. Then they'd have to get it far enough away that the lead could get up to light speed before hitting it's target, you can't do this in solar system because you can't change trajectory that fast. Finally they have to aim it correctly at earth from that distance.
A big ask for a group of people who's best technical achievement so far is to fly a plane into a building.
There seemed to be a measurable effect going on, so they tested it.