About the use of nco with these files, see https://cjcoats.github.io/ioapi/index.html:
I/O API files are not synonymous with “netCDF files” !!
Instead, netCDF is one of four (and a half) distinct lower layers on which the data and metadata structures for I/O API files are currently available; additional lower layers have been incorporated at times in the past, and may very well be incorporated at various times in the future. Attempts to treat the I/O API as “just a data format” have consistently failed in all cases because the persons attempting to do so have not fully understood the I/O API data structures embedded in these files
For summing these files, etc., see M3Tools programs m3combo which does general linear combinations (including sums as a special case), and m3diff which does pairwise sums, differences, normalized differences and other 2-file operations.