In what way were those people wrong?
They weren't wrong, but the criticism was misguided because saying "it's just a bigger iPhone" dismissed the potential and appeal of a quality, large format touch screen. Especially at the time of the original iPad, when even competing phones hadn't yet matched the performance of Apple's touch screens. That said, I don't think the difference between iPad Air and iPad Pro is nearly as dramatic, but for split screen and stylus use the extra real estate will be welcomed. 10" is pretty tight for that sort of thing.