When is Service Master Key Exactly generated?
... but there is another non official blog which says ...


I'd go with the official unless for some reason it's been clearly proven otherwise.


  Should we correct the information in the blog?


It'd be nice, but I'm not sure what the chances would be of that happening. Worth a shot.


  If it is created during a setup then it should not wait for some objects to be created ...


You can look in sys.symmetric_keys and see the service master key (in master) which has a create date and modified date. If you do a fresh install of SQL Server, these will be the same and should be roughly around the time of the first startup as part of the install which would make the official documentation correct.