I don't believe it, it is just stupidly illogical. If someone wants to cheat, they won't do it like this, one people recording any of it will get them a permaban instantly. It is not subtle at all.
Explanations:
1. Probably joined while the match was in progress, and someone fed that Nobushi hard. Bad players like to challenge single defenders in their home zone, and since "ounooor" prevents them from retreating, they usually die there. Nobushi has especially cruel damage if you can not react to her kicks, she can kill someone in 15 seconds from a few bad reads. 10 with a wall. After that, she has smokebomb.
2. Second Nobushi in enemy team/it was there all along, the Nobushi was there, left it there before returned to guard the home zone (probably thought that it is not really necessary to defend it from the weak opponents she got).
3. Some attacks, like Bulwark Counter, Demon's embrace, Predator's mercy, Skewer, special parry punishes etc. have a lockdown which prevents you from activating revenge. Also, it happens if it is M+K that you accidentally start to write in the chat because you press the wrong button, and as long as the chat is active, you will just write RRRRR while they are killing you.