The yellow line is whatever the ground rules of the park says it is. For example, at Fenway there is a vertical yellow line in left center which shows the rightmost part of the Green Monster. A ball that hits right of that line is a home run. A ball that hits the line or left of the line is in play.
Most of the time, yellow lines signify in play and are there to help the umps determine over the fence or not. And let’s be realistic, an ump is running out trying to get closer look but he’s still going to be a couple of hundred feet away trying to make the call on a line drive. In today’s case, there was a clear view (blown up) showing the ball hitting the line. It was ruled correctly by New York.
Umps are better off ruling a ball in play and letting things play out rather than ruling a ball out of play. A call can always be changed after review to a home run or a foul ball with minimal impact.