The short answer is that your global variables will be available with the Option Private Module. Incidentally, "Public" is more common because it's not limited to being declared in a module.
The MS explanation isn't very clear IMO and less so because I don't think that cross referencing excel vba projects is very common. The main practical use of Option Private Module appears to be to remove the subs from the macro list.
Nevertheless, here's how you can demonstrate the effect.
Save and open two xlsm workbooks Book1 and Book2.
Add modules to both.
Change the VBAProjects to VBABook1 and VBABook2 respectively.
In a Book1 module, declare a public constant Public Const x As Integer = 10
In a Book2, create a reference to Book1's vba project (see picture)
In the Book2 module add a sub to show x as a message
You should find that x will not be available to Book2 if Book1 has Option Private Module.
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