I think that it's important to realize that for many hardcore raiders, they have to make a decision either now or as soon as possible in regards to whether they want to play Summoner or Black Mage in raids. There are many players that can play both at a high level but there are also many players that need time to practice and learn a single job, and these next few weeks are extremely valuable time. Of course they'll be able to get some valuable insight from Alexander Normal but that might not necessarily translate over to Alexander Savage.
That being said, I do not disagree with your point in the sense that both Summoner and Black Mages will be completely viable in a raid setting - however, viability and optimization are two very different things. If you choose to play Black Mage and your static is fine with that then you'll have absolutely no problems clearing the new raids assuming that you're raiding at a regular pace.
This Summoner vs Black Mage comparison is much more relevant in a heavy progression setting - some people will be devoting the entire first week of Alexander Savage entirely to Alexander Savage, while the other large majority of raiders will be raiding as much as they can over the first month. If those raiders are caster mains that want to know whether to practice Black Mage or Summoner, then they need to know as soon as possible which job they want to play. Something that will influence this decision is which job is better suited for a heavily undergeared progression environment. These people do not have time to wait and see if Black Mages will be able to handle the new raids. They want to gather as much of the information that we currently have and make a clear decision on which job is better in a blind progression setting.
I believe the document tries its best to analyze both classes, address their advantages and disadvantages, and make a reasonable conclusion on this subject.
A reasonable conclusion to come to is that, in a blind, unknown raid situation, Summoner offers much more adaptability, mobility and have more consistent damage due to multiple damage sources - DoTs, Pets, Ruins, Aetherflow/Dreadwyrm Trance. If one part of a Summoner's damage is performed incorrectly, the rest of his damage will still be going.
Black Mages on the other hand offers little adaptability in blind situations, zero mobility and a singular damage source. If a Black Mage performs his rotation incorrectly, his damage will drop considerably. Furthermore, the fact that their damage scales indirectly with their mobility (less movement = more damage) does not bode well for handling new mechanics.
That being said, if you choose to play Black Mage, then that's your choice. However you might find yourself considerably handicapped in a blind situation when compared to a Summoner of the same amount of skill.