Is main() really start of a C++ program?

You tagged the question as "C" too, then, speaking strictly about C, your initialization should fail as per section 6.7.8 "Initialization" of the ISO C99 standard.

The most relevant in this case seems to be constraint #4 which says:

All the expressions in an initializer for an object that has static storage duration shall be constant expressions or string literals.

So, the answer to your question is that the code is not compliant to the C standard.

You would probably want to remove the "C" tag if you were only interested to the C++ standard.

You are reading the sentence incorrectly.

A program shall contain a global function called main, which is the designated start of the program.

The standard is DEFINING the word "start" for the purposes of the remainder of the standard. It doesn't say that no code executes before main is called. It says that the start of the program is considered to be at the function main.

Your program is compliant. Your program hasn't "started" until main is started. The function is called before your program "starts" according to the definition of "start" in the standard, but that hardly matters. A LOT of code is executed before main is ever called in every program, not just this example.

For the purposes of discussion, your function is executed prior to the 'start' of the program, and that is fully compliant with the standard.

Your program will not link and thus not run unless there is a main. However main() does not cause the start of the execution of the program because objects at file level have constructors that run beforehand and it would be possible to write an entire program that runs its lifetime before main() is reached and let main itself have an empty body.

In reality to enforce this you would have to have one object that is constructed prior to main and its constructor to invoke all the flow of the program.

Look at this:

class Foo
{
public:
   Foo();

 // other stuff
};

Foo foo;

int main()
{
}

The flow of your program would effectively stem from Foo::Foo()

No, C++ does a lot of things to "set the environment" prior to the call of main; however, main is the official start of the "user specified" part of the C++ program.

Some of the environment setup is not controllable (like the initial code to set up std::cout; however, some of the environment is controllable like static global blocks (for initializing static global variables). Note that since you don't have full control prior to main, you don't have full control on the order in which the static blocks get initialized.

After main, your code is conceptually "fully in control" of the program, in the sense that you can both specify the instructions to be performed and the order in which to perform them. Multi-threading can rearrange code execution order; but, you're still in control with C++ because you specified to have sections of code execute (possibly) out-of-order.