Given how upset Matau was, Nokama's and Whenua's lines seemed like they were mainly trying to calm him down. Reassurances that they were still a team and would undo their mutations.
It could've been phrased differently, but I don't think it was so horrible.
As for it being OOC for Whenua, having Nuju or Onewa say it would've been even worse. Onewa was snarky and Nuju was logical, whereas Whenua had more wisdom. It couldn't have been Matau, Nokama, or Vakama, and giving it to Nokama would've meant she was the only one talking (and I feel like the two statements wouldn't have fit well coming from the same person).