I only saw then end of the program so I am not sure if they mentioned this as a possible cause.
The standard theory is that Type Ia supernova are all about the same luminosity as they are caused by the explosion of a white dwarf when it reaches the Chandrasekha mass limit. This holds true if matter is slowly added to the white dwarf so that it goes bang at exactly Chandrasekha mass. Hence a standard mass for the explosion, a standard luminosity, and so you can work out how far away it is.
I’ve heard of the possibility that you could also get a Type Ia supernova by the merger of two white dwarfs. They could have various masses, so the mass and luminosity of a Type Ia would not be a set value. This would cause problems with the determination of the Hubble Constant using this approach. I am not sure if this has now been discounted, or something that is quietly overlooked.