Concerning one point of disagreement concerning policy formulation in the above discussion, I would suggest that the normal approach is:
Yes, knowing something to be untrue, is generally accepted as a reason to tag or remove material. In fact, the extreme argument, which I do not have much sympathy with, is that questioning un-sourced material is always acceptable, no matter what the intention. (But then people generally add some proviso when arguing this extreme version, such as "except when we are talking about obvious behavior issues".)
Trying to formulate what less extreme people think, I would say that believing something to be unlikely to be something that people could come to a consensus about being true is a valid reason for tagging or removing material.
This is perfectly consistent with our real way of working: verifiability, the ability to confirm that something is what rational people could agree to be true, is our threshold for inclusion, but not personal opinions about what is true.