This seems specious. Correctly assigning blame won't matter to readers; most people couldn't care less. Sticking a different brand name on the failure is side-stepping the issue: nothing stays up 100% of the time. Create a custom page that treats the situation with a little bit of levity.

If legitimate downtime happens often enough that someone would actually internalize the difference between your failures and Heroku's, you have bigger problems than your error page.