[Yahoo Mail] became slow and clunky and inherited the worst parts of Gmail and none of the good parts.
Yahoo and LinkedIn are basically right next door to each other and employees from one routinely hop to the other, so it makes sense that they share production values.
You're right, the poorly implemented copies of Gmail features could just as easily been written by Xooglers, but it's a funnier and longer-standing joke that the entirety of LinkedIn is as a hodge-podge of other peoples' ideas.
Google and LinkedIn are swapping buildings, so that will no longer be literally true when the swap is done. But I agree that it's a silly thing to say; Silicon Valley companies do swap employees all of the time.
Yahoo and LinkedIn are basically right next door to each other and employees from one routinely hop to the other, so it makes sense that they share production values.