Yeah, because of the binding, as far as they're concerned, she ought be dead by now. Complete the mission or a magic unbreakable ribbon will slowly cut through your wrist and sever your hand. Which - considering blood poisoning, blood loss, infection etc - is fatal when you can't get treatment.

So the Silvergrove elves think Rayla refused to do the mission and didn't even return. Ethari didn't even comment on her lack of a binding - he doesn't think she was ever wearing them.

One of the other assassins said the mission needed six to work, but didn't explain why six was crucial. So the Silvergrove elves may consider her failure to be the sixth as compromising the mission so much it contributed to the team's death? Or simply bailing at a critical moment as too dishonorable to be an Assassin?