For humans yes. These aliens seemed to have the following tolerance:
Oxygen 0.151 (+/- 0.060)
Shouldn't that mean between 9% and 21% or am I confused here?
There are two different factors at play.
One is the partial pressure of oxygen (or methane for methane-breathers) in ATM. That is the value you're looking at there. 0.151 atm +/- 0.060 atm, so between 0.091 and 0.211 atm.
On top of that, there is a hard cap on the PERCENTAGE of oxygen (or methane). This hard cap is 30%. In the OP, the oxygen percentage is 33%, which makes the atmosphere instantly unbreathable and means there probably isn't enough infra on the body for the existing population, which causes the negative growth rate.