Well, seeing as you never responded to my request for sources and such in the other thread, this is rather a to do, is it not?
But c'est la vie. I shall bite.
First, I have specifically said that what he did is not cheating.
Secondly, boosting is generally considered to be "In multi-player games, having other people assist you in getting leaderboard position or achievements (xbox 360 specific) by having other people help you in ways that go counter to the game's design." Taken from Urban Dictionary, not the most reliable source but I am fairly sure most people would agree on said definition.
With said definition, well, it is obvious, is it not? Not to mention he went AFK at some points during this, making it even less effort, if you wish to call it that, on his part. Oh, but it's a SWAG, you say. And? Shared World Adventure Game. Are you in a shared world sharing the adventure? No. You are server hopping to achieve a goal in game which is generally recognized by the community as a reasonably big deal. You are not engaging in an adventure, you are joining groups purely to turn in loot. There is no adventure, no engagement and no point beyond being the first pirate legend.
We can argue the semantics of it till we are both bleeding from our fingertips, but at the end of the day the only way you can say that boosting does not exist in a SWAG is by, well, your opinion. Which you seem to feel is more important than the communities, who seem to agree that he 'boosted'.
Oh, hey. Know TB? TotalBiscuit? Kind of a big name when it comes to streamers. I don't follow him, nor do I have a twitter account, but a google search for prodigyx and boosting returns this:
"How sad, not only does ‘pirate legend’ status not get you anything interesting other than remixed voyages with the same goals, the first one to accomplish it was a booster who used his twitch followers to power level with what could be argued as an exploit. Sea of Thieves folks"
So there's another opinion from someone who is in the same, I guess? industry as the player. Of course, as with everything else, it is purely opinion, but unless Rare decides to weigh in there is no actual concrete answer. :)