Raw lima beans do contain a compound, a cyanogenic glycoside, which would release cyanide if ingested raw.
Frozen lima beans are typically uncooked. The usual stories about cooking many varieties of lima beans quote boiling for at least 15 minutes and discarding the cooking water.
Now trying to unwind the folklore....
(A) What is the toxicity of cyanide?
Cyanide in the body is an anion, which has a negative charge. In order to balance the charge a cation, a positive charged ion must be present too. Since beans would be eaten let's look at the toxicity of sodium cyanide.
The dose of sodium cyanide which would kill 50% of the subjects (LD-50) is about 15 mg/kg of body weight for mammals. But by weight sodium cyanide is 47% sodium and 53% cyanide. So the lethal dose for cyanide itself would be about 7mg/kg.
So a 154 pound person weighs 70 kg (2.2 lbs per kg). 70*7 is approximately 500 mg or 0.5 grams.
(B) Does the cyanide content of lima beans vary?
Yes. Supposedly the varieties of lima beans consumed in the US (and presumably Canada) are specifically chosen to be low in cyanide.
Cyanide in "lima beans [varies] between 15 and 500 mg HCN/kg fresh weight (Bickerstaff, 2003)" quoted from:
But I can find the original source by Bickerstaff in the article.
(C) How much does cooking reduce the cyanide?
There is a paper which notes that in other foods the cyanogenic glycosides are 100% destroyed by cooking.
(D) How does the bean produce cyanide?
The cyanide in lima beans is present as "pure" cyanide, but a cyanide compound called a cyanogenic glycoside. Lima beans contain two such compounds, linamarin and lotaustralin
I'll quote Dr. G. Speijers of the National Institute of Public Health and Environmental Protection, Laboratory for Toxicology, Bilthoven, The Netherlands
The first aspect is the processing of plant products containing
cyanogenic glycosides. When the edible parts of the plants are
macerated, the catabolic intracellular enzyme ß-glucosidase can be
released, coming into contact with the glycosides. This enzyme
hydrolyzes the cyanogenic glycosides to produce hydrogen cyanide and
glucose and ketones or benzaldehyde.
It seems that the human gut also has bacteria which are capable of hydrolyzing the cyanogenic glycosides to produce hydrogen cyanide.