I think they're talking about the 2nd-derivative of the cumulative deaths. As they can (presumably) only increase, the first derivative is >= 0. The second derivative tends to zero as the cumulative deaths flattens off.

Edit: Now that I think about this some more, you're right. There has to be a decrease in the daily cases, which implies a negative 2nd-derivative. They mention "relative 2nd-derivative." It seems to be defined in the paper in a way that leaves me more confused:

> Daily fatality rates from the included countries were then used to calculate estimates of the relative second derivative of total deaths, N, 1/N d²N/dt², for a period of at least ten days.

Does this mean they are taking the second derivative of the reciprocal of the cumulative deaths?