I feel like they don't know what pivoting means. The first pivot doesn't sound like a pivot; they built a new business with the same venture money. The second pivot isn't a pivot either. I think reasonable people would call that "growth and expansion." Pivoting is supposed to mean that you changed strategic direction based on validated learning. Twitter and Instagram started as pivots. This sounds nothing like that.
However w/r/t The first pivot doesn't sound like a pivot; they built a new business with the same venture money.
That sounds like a pivot, the way I define it. I agree most pivots do have some tangential relationship from the previous goal thats because you learn where the market is from what you've done. Thats not to say its the only way to find a new market opportunity. I'm not sure why you define Twitter as a pivot and not Fab. Twitter was Odeo which was a podcasting company which was sinking. Evan Williams had blogging experience but Jack Dorsey (as I understand it) was the original idea guy behind Twitter.
A lot of pivots aren't related to the original idea that founded the company. For the VCs doing something else with that money is almost more important then just continuing to chase an idea without legs, since once the money is invested they can't reinvest in new ventures.