It's not clear to me that "development tier" means "a development environment." I interpreted it as the standard StackOverflow.com infrastructure that developer

It's not clear to me that "development tier" means "a development environment." I interpreted it as the standard StackOverflow.com infrastructure that developers use -- this is contextualized a few paragraphs later when they mention "we maintain separate infrastructure and networks for clients of our Teams, Business, and Enterprise products", which implies that the production environment that was compromised was something other than those three. (Even if I'm right, though, I definitely agree that the wording is very easy to misinterpret and they should clarify the post to explain what the "development tier" is.)