Gold Cup horses covers a very wide range of ability.
Lord Windermere was one, yet he was no better than a very early 160s horse. Chances are the nags Arkle beat that went onto GC success were in that range. Is that a guess? A little. However since the early 90s there have been plenty of below 170 Gold Cup winners. In the 1960s with far fewer horses in the population those outliers would have been more commonplace. Stands to reason.
That makes Arkle a mid 190s horse, no better.
And don't forget, Phil Bull had no interest in Jumps racing so his best guys wouldn't have been handicapping the winter game.