That statement is patently absurd.
Ray Bradbury, who sadly died today, and generations of writers prior to the advent of personal computers seemed to have managed quite well. Bradbury wrote a novel on paper using a fax machine to send pieces to his co-writer, laying then out on the floor to organize. There are also thousands of writers who still write without a computer. Is a proper sitting position "in a booth at a coffee shop?" If not, we might have had no Harry Potter.
Saying that one explicitly needs a desktop system (with the parenthetical insinuation that a laptop is possibly substandard) is ridiculous.
Plenty of people find the iPad an exceptional interface for writing. One with a fewer number of distractions than a full-fledged computer. In fact, with the Bluetooth keyboard, it very much reminds me of the word processors I used decades ago- a distraction-less environment that's actually very conducive to writing. Don't listen to someone telling you that an iPad is not a good writing interface, try it out and see if it works for you. It's a new technology and many, if not all, writers will incorporate it as they have all new technologies.