Seems like soon you will be able to show numbers of how many more people didnt seek treatment because they looked at some app and thought "Now I know CBT. Its crap". Please make sure to publish those numbers.
It is also worth noting that this is not a new phenomena. The same could be true of people buying CBT textbooks, like the famous "Thinking good, feeling better" and other titles from the same author Paul Stallard, which have been around for many years. Maybe some people who bought that book got nothing out of it and were put off seeking therapy because they read it. There is little definitive hard data on the subject, but we do know that both app-based CBT and bibliotheraphy can both be effective (citations in original post).
If a user churns, there are many confounding variable associated with the loss of that user. You do not know the exact reason why they stopped using the app, and whether the impression of the app influenced their overall view of CBT in a positive or negative manner. Additionally, since durable recovery rates from in-person CBT are around 50% (citation elsewhere in this comments thread), you do not know if that user was in the category who would've benefited from CBT in the first place. They may have got a similar impression had they seen an in-person CBT therapist. Short of capturing the user post-churn and asking them in person, it is difficult to get a firm handle on such figures, but it's certainly an interesting and important statistic which hopefully will become clearer as research on the area progresses.