I have spoken English all my life and I am not familiar with this saying - So it is probably quite new - and may have a small geographic usage.
That having been said I will tell you what it suggests to me.
Dope is Marijuana - usually the product of a variety of Indian Hemp
Rope for centuries was made from the very strong fibres of Hemp - but not the same variety that is smoked as marijuana.
My thought is that we have two very different products arising from a similar source. In other words it is a way of stressing the difference between two things - They are as different as Dope and Rope.
When I was a child we made a similar comparison with chalk and cheese.
e.g. "They may be brothers, but they are as different as chalk and cheese"
or shorter - "Bob and Tom are like chalk and cheese"
I suspect that Dope and Rope is being offered in a similar way
I hope this is helpful, or if I am wrong that someone with a more intimate knowledge of the term will provide you a more accurate answer.