The tree is foundation of this country. It favors a small selection of people it marks. This gives that selection of people additional powers beyond magic. With it this doofus was able to beat up one of Marie's chucklefucks (who are VERY good at fighting) like it was nothing.
This creates your typical scenario from proverbial dating sim of terrible nobles lording over the heroine with their bullshit power. It's just that this time MC group is on the recieving end.
And yeah, it is INCREDIBLY unfair. Even in manga characters are stumped that it worked because they themselves said it was not theirs, but the tree has it's favourites.
The reason that a rando can't get the whole Republic is because the rest of the elite are also tree favorites so they are more likely to loose if they tangle with those.
Yes, this is veeeeeeeeeerrry heavy handed, but a typical trope in these kind of settings.
The one thing to consider is, why was Luxion included in the bargain. Not like the boat in question is Luxion's main unit. Nor did Piere and his crowd know of his existence.