I guess that bit is unclear and would make a big difference from the perspective of the brother.

OP: I want to offer this laptop so that my nephew has a rig to play on.

Brother hears: I am giving you this machine so your son can play Minecraft on that silver POS you've been working on.

It doesn't seem that disjointed and weird to me. It seems like a normal miscommunication about the intent of a gift, versus its use.

Was the black laptop seen as a family gift or a gift for the nephew? We swap around machines all the time in my household according to what's handy and who's running what. Is the silver one really too slow for minecraft? Does the dad really never let the boy play on the black machine?

I really didn't see anywhere that the OP clearly stated the gift was to be presented solely to the child or that OP presented the child with the black machine himself. Just that he gave the black machine to his brother, specifying the goal of providing the boy a machine to play minecraft on (which the brother might well could fulfill by swapping out his old machine).

I know it seems like OP had been more explicit than that, but he says twice he gave it to the brother, not the nephew. It very well could be confusion around whether it was a household gift to benefit the child or an individual gift to the child himself. The distinction is important in a household, and with large items like a laptop especially, the distinction should be made explicitly by the giver and communicated upon giving.

Did OP wrap it and label or present it to the kid? If not, it seems like a pretty innocent mistake that could be being blown out of proportion by the kid and a disappointed uncle.

I dunno. Maybe bro doesn't deserve the benefit of the doubt. I know few fathers who will take an individual gift away from their child, but most would work a new family machine into the technology rotation however the family handles such things.