Why is it correct to say that nothing concrete regarding the arms race was achieved at the Geneva Summit of 1955?
It is correct to say that nothing concrete happened on arms control because there were no specific agreements reached.
At this meeting, the most important proposal on arms control was President Eisenhower's "Open Skies" proposal. That plan called for both the US and the USSR to allow spy planes to fly over their territory so that each side could verify that the other side was not planning to attack the other (because at this time there were no ICBMs and an attack would have involved masses of bombers). Eisenhower believed that this would allow each side to fear the other less and would eventually lead to arms control agreements.
Nothing concrete came of this because the Soviets rejected this proposal. They did not trust the US and were certain that the proposal was a trick.