I don't understand the underlined part, could anyone help me out?
Thank you in advance.
"It's unbelievable that John and Mary made it this long. They went together like whipped cream and sardines. That and the speed of it."
Just guessing, but I'd say it means:
That (refers to the opinion that they were quite opposite from each other, like whipped cream and sardines) and the speed of it (how fast the relationship developed.