Actually the World Hard Court Championships was considered the superior tournament to the French Championships because all the top players usually competed there. The French was closed to foreigners till 1925. The WHCC was a major event of that time, therefore, I think, Suzanne deserves those wins to count as slam wins. She would have defeated all those players in Paris anyway.
What you say is very true, although the (Closed) French Championships were open to foreign members of French tennis clubs. The first winner of the men's singles title at that particular tournament, in 1891, was an Englishman, the elusive Mr Briggs. The American player Virginia McVeagh was runner-up in the women's singles event in 1906, while Elizabeth Ryan, a native of California, took part in the Closed French Championships in 1913.
What "SamL" says about indoor tennis being contrary to the spirit and history of the sport, doesn't make sense. The first British Covered Court Championships tournament took place at the Hyde Park Club in London as early as 1885. There were also indoor facilities very early on in places such as Brighton/Hove and Dublin, Ireland.
Given the climate in the British Isles, indoor tennis was inevitable, if not logical.