Hi! Paul here-- you've happened upon one of the perhaps thorniest questions inherent in that poster and in our series: "Where Were the Middle Ages?" (thread)
First, a note about the poster-- as I hope is clear, the poster is meant to be a necessary simplification of some very complex and nuanced ideas in order to "shake up" or "break the ice" on some of the calcified ways that the public thinks about the medieval past.
It's not meant to be a be-all and end-all; it's a place to start, rather than a place to finish. A teaching tool, rather than a scholarly tome.
Speaking of places to start, a great one for the question you raise about Where the Middle Ages Were is @MarianneOD1's excellent essay for our series on that subject: publicmedievalist.com/where-middle-a…
The problem is that the Middle Ages necessarily has some geographical/cultural as well as temporal boundaries as many historical "epochs" do, but in my experience there is little or no agreement over where those lay even among experts.
If we defer to the originators of the idea, the Italian humanists, it is based upon cultural markers that are European/Mediterranean. But we don't need to defer to them; I think we shouldn't.
Not least because it tends to present a world that was trapped in amber-- one in which Europe stood as a defined and more-or-less homogeneous thing, typically in stark contradistinction to the Dar al Islam, sub-Saharan Africa, or Eastern and Southern Asia.
But it wasn't. Not culturally, not materially, and certainly not "racially."
The boundaries between "European" and "non-European" were fluid and fuzzy to the point of being practically nonsense, as peoples moved and mingled, ideas and objects spread.
Imposing a modern political map backwards onto the past is inaccurate (and incidentally a common bit of sleight-of-hand done by white supremacists).
So where does that leave us? I find that people (scholars and amateurs alike) tend to define "medieval" geographically based upon cultural and temporal reference points that they find to be important.
My personal definition centers on the idea of "spheres of influence" around centers of cultural and political power-- which I got from my interview with @kusimbachapuruk publicmedievalist.com/recovering-med…
But it's something I'm willing to concede that reasonable scholars can differ about. But the purpose of the statement on the poster was a broadside against the white-supremacist-laced idea that the Middle Ages were exclusively and definitively European (read: white). /thread
