Yes, that is true. Although there are concerns that an increase demand in the market will lead to crops being grown for bioplastics as with bio-fuels. However, according to most of the producers of compostable plastics, the corn starch and sugar cane ethanol is extracted as a bi-product of the crop production, so the crop is being harvested for multiple purposes e.g. corn for animal production and sugar cane for sugar.
One of the issues with these materials is that they can’t be certified organic or GMO free because of the crops they come from. Another issue, previously mentioned is that even though they are plant-based they could still cause damage to animals as they won’t degrade if they become litter.