Why is Smalltalk OOP, but C++ not (according to Alan Kay's definition)?

I know that Alan Kay is the one who coined the term Object Oriented Programming and I know that Smalltalk is considered to be one of the languages that falls within the definition of Alan Kay's OOP. I heard Jonathan Blow talking about how C++ is not really an OOP language in a sense that it doesn't comply with the definition of OOP by Alan Kay.

Is it possible for anyone to explain this? Any references are welcomed as well. Thanks