Principles Not Products

I reviewed about SQL & Relational Theory Master Class. Some idea what I interest, Principles Not Products. He said Database Professional need to know the relational model and the relational model isn't product specific. When we change from some environment to the environment somewhere else, we don't need to transfer knowledge. How do you think?

On the real world ,we will see many databases (Oracle, DB2, SQL,...), which each of database uses the same theory. So, we should know the theory also(Example: SQL & Relational Theory Master Class).