It's not mind reading when the President himself has expressed animosity towards Amazon & Jeff Bezos. Comb through his twitter feed for amble examples.
As for someone like Mattis, he has shown nothing in his career that I'm aware of that undermines his integrity and desire to serve his country. Yes, he may bear some ill will towards the president, but at least from an outsider view they parted over legitimate policy differences, and there's no reason to believe Mattis would have outright fabricated a quote like "screw Amazon". If we were talking about someone else (maybe John Bolton) I would be more skeptical. Mattis... I'm not saying it's %100, but I'd be more likely than not to believe he was honest in his recounting of that quote. That quote aside, instances where President Trump has bashed Amazon & Bezos are common-- again, the President's twitter feed is a good record on this topic.
This is not the type of circular reasoning you refer to, but I acknowledge that can be an problem... one person speculates, another news source reports on it, a third uses both the first two to paint a picture of consensus. That Mattis quote and President Trumps direct statements elsewhere don't fit that mold.
But lets link it back to the main point at hand: Could even the directly attributable statements from President Trump be taken as involvement, or as potentially influencing the procurement process? That is something reasonable people might disagree about. That is why there is this legal fight. At this point, there is simply too little publicly available information to say "yes" or "no". There is enough to suggest the possibility. The discovery portion of pre-trial will have to investigate to gather more evidence, or fail to do so and not prevail. Currently there's enough to suggest the possibility that a court agrees a case can move forward, but it's important to note this isn't some partisan "never Trumper" decision. The court was still skeptical enough to require nearly $50,000,000 be set aside against the possibility they are wrong and therefore need to cover damages resulting from the suit & project delay.
Regardless, I think we've both said our piece here. I've clarified as best I can, as have you. I don't see much to be gained by going forward. I'll say only that I disagree with your assessment of the situation, but appreciate a dissenting point of to make me think more critically about the topic, and hope you can appreciate that my own opinions come from a place of honest opinion, not some partisan position (I don't fit the mold on either side, and am generally criticized by people on both sides when I don't reflexively support "the party line.) So, thanks for a thought provoking thread.