Yes, The US Open, Wimbledon, and Australia were all played on grass. What impresses me about the grand slams Laver won is that they are seven years apart. Laver, Rosewall, and others were banned from the slams because they were playing professional events for little money in these out-of-the-way places. When the US Open became the first tennis event of the Open era in 1968, the first full year of Open tennis was 1969.
Laver probably spent the best years of his career toiling in obscurity, therefore we don't know how dominant he would have been because he could not play those events. I cannot take anything away from Roy Emerson, but Laver would have won many of the slams he accumulated and Laver could have won 20+ slams if allowed.