More technical details. For the time dilation you must correct the time by 1/sqrt(1-(v/c)²) that can be approximate for small speeds as 1+½(v/c)².
If the speed of the neutrons is less than 1e-5 c, the correction is less than 1e-10.
The lifetime of neutrons is approximately 15 minutes lifetime, so the correction is less than 1e-7 seconds. But they are measuring with a precision of only only a few tenths of a second, so the corrections is negligible.
Comments
More technical details. For the time dilation you must correct the time by 1/sqrt(1-(v/c)²) that can be approximate for small speeds as 1+½(v/c)².
If the speed of the neutrons is less than 1e-5 c, the correction is less than 1e-10.
The lifetime of neutrons is approximately 15 minutes lifetime, so the correction is less than 1e-7 seconds. But they are measuring with a precision of only only a few tenths of a second, so the corrections is negligible.