Maybe it is mostly FUD, but it does not mean it is impossible. Yes, soy-based plastics are very processed, but it does not mean that the end product has no impurities that "smell" like soy. Rodents have very sensitive smell and some can even detect landmines. Also, plastics made solely from biomaterial or with bio-based components (e.g. plasticizers) are not (always) chemically exactly similar to petroleum-based plastics.
I found one research article from 2020 titled "Assessing Rodent Gnawing of Elastomers Containing Soybean Oil Derivatives". It did not find statistically significant difference in rodents gnawing when soy oil derivates were added to plastics. Maybe they have found a way to remove the tasty components.
I found one research article from 2020 titled "Assessing Rodent Gnawing of Elastomers Containing Soybean Oil Derivatives". It did not find statistically significant difference in rodents gnawing when soy oil derivates were added to plastics. Maybe they have found a way to remove the tasty components.
https://pubs.acs.org/doi/10.1021/acssuschemeng.0c05868