If it is a myth, it is a very widespread stubbornly persistent myth. If you have links to scientific sources debunking it as a myth that would much appreciated.

I came across it most recently in Tom Brady's athletic health book [0]. If I remember correctly, he states that you should drink double the amount of coffee consumed to offset the dehydration effects of coffee. This is simply to emphasize the ubiquitous nature of the myth, as he is someone who has presumably been surrounded by medical professionals for most of his athletic career, and who should have known better if what you say is correct. He singles out proper hydration as a major factor in his peak performance, something he claims younger athletes ignore.

[0] https://www.amazon.com/TB12-Method-Lifetime-Sustained-Perfor...