C is a classic low-level procedural programming language while C++ is a superset of C that is both procedural and object-oriented. Both C and C++ are commonly used languages and though C++ is derived of C both languages need to be approached differently.
I have no idea where you got that quote, but it's simply not true.
The rules surrounding unions being different like we discussed is one of the reasons why C++ is not a superset of C. Code that uses unions for type punning is valid C, but not valid C++.
There are many other subtle differences as well, different keywords, declarations ... I was going to include them here, but you can just google it.