That's not saying it was undetectable that's saying we are not sure it's been detected. In other words looking at the same data in the future with better models of what's going on it may be clearly detectable with our data. However, we don't feel confident enough to say it's been detected.
Also, it should be noted that the current administration is very hesitant about damage from global warming so this could just be weasel words for any significance can always fail the 0.0000...1 significance test.
This is unrelated to the current administration. "No detectable change in storm" has been the finding for at least a decade. (I just checked AR4 published in 2007.) Your allegation of using more stringent test is also groundless.
Sure, but the same was said in 1980 and before. Still, climate change continues and we are collecting more data.
So, there will be a transition point. In the end there are two forces, one is the random nature of weather which adds variability with any change. Another is the increase in storm strength from increasing temperature.
So, we are already at the point where strong storms are statistically more likely. However, at what point can rule out randomly getting lucky such that existing storms have not gotten worse due to random luck?
That question of what is considered enough has become a political question, so you can't expect the transition point to be chosen on a purely empirical basis.
Also, it should be noted that the current administration is very hesitant about damage from global warming so this could just be weasel words for any significance can always fail the 0.0000...1 significance test.