Yes, that's correct. Give those limas as much distance as you possibly can. Another thing which may help is to actually use other beans as barriers. For instance, you could plant Insuk's Wang Kong between the regular beans or limas, adding a little more protection. I've grown limas for well over 20 years and never had a single cross. But then, sometimes I only grow one variety in a given year. Other times I've given them all the distance I could, besides surrounding them with another species of bean. The only time I've had a cross between regular beans (vulgaris varieties) was early on when I didn't know enough, and I planted them together.

George
Tahlequah, OK

PS. Here's an interesting anecdote. The first year I grew Flossie Powell Pole lima, we were in a region in Mexico where limas were unknown. It was my only lima that year. So I calculated that I had approximately 50 miles of isolation distance!