Your post is full of great information but didn't need the heading. Odds are really good that the poster you are replying to isn't actually lying. Instead they just aren't as familiar with DT as you are. You could have imparted the same information beginning with an opening that educated the user instead of insisting they were lying and a demand for proof.
Firstly calling someone a liar basically shuts down all discussion in most cases and secondly after you have provided the proof you needn't demand it of them. You can instead wait for them to absorb the data you have provided.
Example:
> Darktable in fact has the features you mentioned and more. I enjoy using it for professional work myself so let me tell you a little bit about the features that make it competitive.
This would refute the position without directly engaging in negativity which is more likely to be read especially by poster but I think we are all a bit tired of people calling each other liars on the internet at this point.
Shutting down the discussion should be the priority when one participant is just confidently asserting misinformation. Their motivations for doing so are not of interest.
Firstly calling someone a liar basically shuts down all discussion in most cases and secondly after you have provided the proof you needn't demand it of them. You can instead wait for them to absorb the data you have provided.
Example:
> Darktable in fact has the features you mentioned and more. I enjoy using it for professional work myself so let me tell you a little bit about the features that make it competitive.
This would refute the position without directly engaging in negativity which is more likely to be read especially by poster but I think we are all a bit tired of people calling each other liars on the internet at this point.