So because one cannot know which type of people submit data to the site it should be disregarded? That’s basically saying any poll or questionnaire with anonymous yet unique answers are invalid. That’s a pretty bad argument.

This is basically a survey or poll. You want people to provide you with data about what they're running. To get an accurate view of the entire population you need a representative and randomized sample. If you're relying entirely on self-reported data you're not going to be getting a reliably randomized subset of people. You'll get people who are motivated to report their usage to a third party. That can lead to persistent biases in the data.

It may be that Wayland use is being under represented because the people reporting want to show that "X11 is still king!" Or it could be that this website is shared frequently with certain user groups (e.g. in some arch (btw) forum or something) and so you're getting a skew towards that population and away from the whole.

We don't know who these users are and we can't "offset" for those factors. And the data isn't reliably randomized so it's subject to those biases whether we know about them or not.

Though as another person pointed out the trend itself may be of some interest if the population being polled is consistent. Though I doubt anybody suspected that Wayland use is NOT increasing?

Anonymous polls are indeed useless for several reasons.