Not a great start in my opinion:
A relational database contains tables that relate to each other by key values
Relational databases fundamentally have nothing to do with tables being related to each other. The topic is allegedly mathematical so it's a pity the piece couldn't begin with an accurate and informative explanation of what the relational model is about. Someone who doesn't know any better is going to be misled into thinking that "relational" is all about "relating tables".
What bothers me even more are the examples. What is the point of demonstrating tables without keys, with duplicate rows and with every column nullable?? These are not sets at all.