Let me rephrase GP into (I hope) a more useful analogy. — actually, here’s the whole analogous exchange:

“A rectangle is an equal-sided rectangle (i.e. “square”) though. That’s what the R stands for.”

“No? Why would you think a rectangle is a square?”

Just as not all rectangles are squares (squares are a specific subset of rectangles), not all datagram protocols are UDP (UDP is just one particular datagram protocol).