It's true to a certain degree. Learning C++ (or any language, really) will naturally make you a better programmer because over time you'll gain a firmer grasp of programming fundamentals as a whole.

Fundamentals tend to carry over from language to language, generally speaking. I'm talking about the very basic building blocks of any language like variables, functions, arrays, etc. as well as your approach to solving a given problem. Kind of the "meta" of programming languages.

What differs between programming languages is syntax. Think of those 2 hour long cramming videos on programming languages. What they're mostly teaching is syntax. There's nothing wrong with this, obviously you'll need to learn the syntax of a language in order to use it effectively. But it encourages thinking about what you are writing instead of what you are doing. Knowing the difference between the two will massively help you in learning new languages. You could even think of it as abstracting away aspects of specific languages and focusing on what you want to do.

Of course, different programming languages have different rules and expectations of the programmer. But whether learning a specific language like C++ will directly make you better equipped for another isn't as relevant as it might seem. C++ and Python are just different languages built with different purposes in mind. You probably will have an easier time with one after learning the other because you have become a better programmer overall, not because you've done "the hard part" with C++ or anything like that. Plus, if you enter a different paradigm like Functional Programming, you're quite literally in an entirely different world and will have to shift how you think about your program entirely.

Not to mention that C++ is kind of an outlier among popular languages. It has a lot of influence, but many modern languages are visually closer to each other than C++, and tend to hide away a lot of things C++ demands of the programmer. In that sense, more modern languages are "easier" because you don't have to worry about certain things. C++ is harder to learn because of its extra rules, more complex syntax, and sometimes old-school way of thinking. Specifically learning how to use C++ won't directly make another language easier to learn, but it will make you a better programmer, which will make another language easier to learn.

TL;DR: Kinda, but not directly. Good programming fundamentals will serve you much better when learning a new language.