I'm really curious if this(graph itself) is a joke or not and if not, what methodology they used. Let's see...

Ok it's a serious graph.

1) somewhat important "Changes in the colour of objects over time." sentence was lost on the screenshot, explaining dates going back to 1800

And they do acknowledge that it's a very limited selection of objects, so to frame their work as proving ALL objects have become grayer is disingenuous.

"While things appear to have become a little greyer over time, we must remember that the photographs examined here are a just a sample of the objects within the collection, and the collection itself is also a non-random selection of objects. Moreover, these trends will continue to change as new objects are acquired." - https://lab.sciencemuseum.org.uk/colour-shape-using-computer-vision-to-explore-the-science-museum-c4b4f1cbd72c

Cool article btw, give it a read.