Primus doesn't stop to relax until he's climbed up the rope ladder and in the treehouse, his whole body tensed up until he sees the two other TCPs okay and safe. Buddy almost jumps out to greet him, but he assures it he'll join them in a moment. He just wants to check the knife first.
It's sharp, and Primus nearly cuts himself on the blade. Primus notes to himself that it might even be sharper than Buddy's knives- definitely sharper with those caps on it. It makes him uneasy to look at, and the feeling only gets worse when he reads the note.
He holds the knife carefully in his hands, weighing it and scrutinizing every part of it. It was small, but with how scary that blade was, that fact only made him feel nervous. Losing track of this thing could get someone seriously hurt. The color of the handle seems to match Buddy's outfit, and the box's colors add to the feeling that it was made for the knife TCP itself.
He brings this up, and Buddy reluctantly says it'll carry it. Primus tells it that it doesnt have to, but Buddy insists that whatever left it here, probably intended the knife to go to it. Plus, with the caps on, Buddy feels kind of defenseless. It sounds like it's trying to force a bit of positivity when it says that it might end up being more useful carrying this thing around.