This is not the impression I got from reading the codex so I guess it depends on how you view the material. But I would say that all Chapters (including the First Founding) venerate Guilliman as the author of the Codex Astartes and the unifier of the Imperium, they all most certainly have a high respect for the Ultramarines as well. Guilliman is with out a doubt honoured through out the Adeptus Astartes, which probably is unique amongst the Primarchs.
In the end it isn't really anything you have to buy into too much if you don't want to, like Gornon said, make your pick.
It is also a question on how the direction of the Codex background is steered by the company as a whole. All of the 5th edition codices make sure to contain a lot of "epic" victories and incredible feats so I'm not sure you can blame such occurrences on specific authors,more a trend of "wow, aren't we cool! Buy and play us!".
Well I do wish I could interpret it the way you do, but to me it's so ham-fisted that it leaves out any other interpretation than (I'll quote it again for you although someone already did):
"The Ultramarines are undoubtedly the best Space Marines ever. Yes, really!....With a few fringe exceptions who have severe mutations (Blood Angels) or stolid stubbornness (Space Wolves and Dark Angels) all Space Marine Chapters want to be like the Ultramarines and recognize Marneus Calgar as their spiritual liege."
And that's straight from the horse's mouth. You see, other chapters
admiring the Ultramarines is fine. For them to be depressed because they can never
be Ultramarines is most definitely not. I'm sure the Imperial Fists are too busy ruminating on how they had the nearly unmatched honor of defending the Imperial Palace during the Heresy to be crying about not being Ultras and plastering their walls with pinups of Roboute Guilliman. Yes they embrace the Codex Astartes, but they have also always (until Matt Ward) looked up to Rogal Dorn a
hell of a lot more than Guilliman. He actually implies that their chapter cult would be worshipping the guy rather than (or at least as much as) their
own Primarch.
That's just an example, the same goes for all the other First Founding chapters and their successors.
It even violates the rule of cool because no one who plays any of those chapters wants to imagine their Marines getting home from battle, locking themselves in their rooms, sobbing and cutting themselves because "they will never be Ultramarines".
Oh and as far as Mephiston, whoever asked why I think stats are tied to physical size: Strength and Toughness have always been tied closely to physical size. Why do Grots have S2/T2? Because they're small. Why
don't Tau have S2? Because they're human-sized. If Marines weren't genetically modified
and bigger than humans they could never have S4/T4. The threshold for your Lord becoming a Daemon Prince (large model) in the 3rd/4th Ed. Chaos Dex was having T5.
I could go on, but anyway you get the gist. Strength and Toughness are physical stats. A very, very few Marine special characters since 3rd Ed. have had a 5 for one or the other, but 6 for both is absurd. Not broken game-wise, but stupid and inconsistent.
I should add that I do not hate Matt Ward as a person, I'm sure he may be a very fine and amiable guy, I just hope that he either stops writing codexes or gets a lot better at it soon.