The actual rule is that coaches, or other players besides another runner, may not physically assist a runner during a LIVE ball. After an over-the-fence home run, the ball is DEAD. So the rule doesn't even apply here. But, even if it did...
Slapping hands to congratulate someone is NOT assisting them in running the bases. It doesn't stop their advance when they shouldn't be advancing, it doesn't encourage them to advance when they otherwise wouldn't, it doesn't aid them in touching a base, or alter their path, or help them to regain their footing. In short, it offers no advantage or assistance to a legally advancing runner.