I hadn’t even gotten as far as figuring-out what all of the labels in this diagram are actually referencing, but it does appear that the second row of pin labels (going from inside -> out) are those from the RasPi. I’m not much of a RasPi user, but all of the other signal names on graphics on the web generally match those in these two columns of the Seeed graphic.
It seems very safe to assume that they’re using the RasPi signal names/labels in this image, which is definitely counter-intuitive. And since there’s no key as to what any of the colors or columns mean, one can only guess or try to figure it out yourself – which isn’t the point of good reference documentation.
Your experimental evidence and a review of the RasPi pinout concurs with your conclusion – that the labels are wrong based on most users’ expectation, without some other explanatory text in the image.
See also 40-pin header layout graphic wrong (backwards!) where I just posted that the entire header orientation is wrong with respect to the other features of the picture.
It’s small wonder you were able to find TX and RX at all…