Do you really think Zenefits just chose this "lesser" method on a whim? I've never met the team from Zenefits, so I can't say for certain, but if they're not idiots then I think there's a reason why they decided not to use the regular API. Whether that's a requirement of getting their app approved by the ADP Marketplace, or some restriction on the data that they are able to get through the official API - I think that there's something that ADP isn't mentioning.
> It sounds like ADP did what any company, large or small, would do when faced with a third party accessing its systems in an unauthorized and inefficient manner.
As far as unauthorized methods go - you'd patch the hole in your security to prevent them from getting through. Unauthorized methods of accessing your systems shouldn't exist, and blacklisting the domain name of the third-party isn't how you fix a security flaw. For the excessive load, they should just implement rate limiting across their system & return a 429 when someone goes over.
Of course, I really doubt that they were accessing ADP via a real security flaw - they were probably just making the same requests as users are able to, but in an automated fashion. And if that's the case then the term "unauthorized" doesn't apply, because users are clearly authorized to make those requests.