That second baseman isn't really a second baseman. He is a utility player that plays most positions, including pitcher. He has been around the majors since 2008, accumulating just over 500 ABs. He is listed at 5'9" and 225 lbs, but that weight has to be off by at least 50 lbs.
Here's a longer look at the play. To his credit (or maybe discredit), the 2B knew he did not tag Nimmo with the ball in his glove and he frantically motioned to throw the ball back to second base, but that tipped off the umpire that the tag was not made, which was clear on the replay. Mets have had 15 of 20 reviewed plays overturned this year, which leads the majors, but it just goes to show that even in MLB umpires make mistakes. Sure, fans (and parents!) want every call to be correct, but many of these are split second decisions. Robot umps, anyone?