My guess would be that they don't want to have that email accidentally used, but they would have a check in the codepath anyway, because no-one wants to see its logs spammed with myriads DNS errors when this can be avoided. And in fact, if a DNS error shows up in the logs, devs would know that somehow their code path is not completely safe, so that change on the email is perhaps a way for them to ensure that the disabled account is indeed seen as disabled by their code in every situation.

A lot of people are complaining about NYT approach, but perhaps their only fault - if one consider that not deleting for good an account is not an issue, and it seems to be a common practice in the industry - is to not use a transaction when disabling user accounts (disable email -> disable account), which is perhaps difficult with NoSQL setups?