Its not a bubble because its value don't seem to skyrocket and pop. It only bubbles due to its infinite supply but the value keeps on inflating and not backed by anything. Bitcoin is the real money with proof and hard to faked fiat money should be inspired by it.
I think they only believe that it was bubble cause they believe that when it become huge like a bubble or I mean the price will be more expensive then it will die sooner I think that is they want to say for that bitcoin is like a bubble.
You can't predict a future but you can make a theory on it so it is depend on a people if they believe on it or not.