Just how much cyanide can be released from potassium ferricyanide depends on several factors, I quote from the following site www.cyanidecode.org/cyanide. One important point is "mildly acidic conditions." Another is how long the contact with an acidic environment lasts. I have personally noticed a definite odor of HCN from a ferricyanide solution that was a few days old. One needs to be careful with all chemicals but some require a bit more care.
"Cyanide forms complexes with gold, mercury, cobalt and iron that are very stable even under mildly acidic conditions. However, both ferro- and ferricyanides decompose to release free cyanide when exposed to direct ultraviolet light in aqueous solutions. This decomposition process is reversed in the dark. The stability of cyanide salts and complexes is pH dependent, and therefore, their potential environmental impacts and interactions (i.e. their acute or chronic effects, attenuation and re-release) can vary.
Metal cyanide complexes also form salt - type compounds with alkali or heavy metal cations, such as potassium ferrocyanide (K4Fe(CN)6) or copper ferrocyanide (Cu2[Fe(CN)6]), the solubility of which varies with the metal cyanide and the cation. Nearly all alkali salts of iron cyanides are very soluble, upon dissolution these double salts dissociate and the liberated metal cyanide complex can produce free cyanide. Heavy metal salts of iron cyanides form insoluble precipitates at certain pH levels."
The recipe given by the OP lacks necessary detail in how the solution can be prepared safely. The nitric acid should be well diluted before it is added to the ferricyanide solution to prevent a localized low pH which could cause problams.