You can do each boss either in hardmode OR nightmare mode as you choose. So you can pick to do ADXRR-3 in hardmode, then do Gharj in nightmare, then switch the operation back to hardmode for Ancient Pylons, for example. But you can only do one or the other - if you've done 8-man Soa on hard you'll have to wait until next week to try him on nightmare.
Also, 8 and 16 mans share a lockout, so you can only do one or the other. These reported kills were all 16 man, and we're locked out for 8-man operations until lockouts reset.