Well, speaking as a Scot (albeit currently working in Norway) I have no concerns with Hamish Henderson or Ewan McColl being cited as major Scots cultural icons of the 20th Century. Nor would I necessarily quibble with Hugh MacDiarmid's inclusion either.
But there are so many to choose from: Robin Jenkins, Ken Currie, Neil Gunn, Sorley MacLean and Martyn Bennett to name just a few that have popped into my head. We're fortunate to have had such riches to choose from.