In MySQL lesson the explanation of Relational Database is incorrect. I know this is a beginner course but still go to be correct.
As per the Oxford dictionary, a relational database is "a database structured to recognize relations between stored items of information." This is exactly what I am saying in the course. You are referring to relational database mathematical theory, which in essence say that and then goes into details on the relational database theory, and how databases are updated and affected by updates.
I think you are reading more into the statement than what is there, and, yes, it is a beginner course, so relational database theory is way outside the scope of the course.