Ice cream IS basic grocery. It's listed under Dairy product section, together with cream, cheese, yogurt (and a few others). The $20 limit do applies. I have my Nexus tariff guide so it's all listed there.
The tariff guide for Nexus user shows that ice creme is under Dairy product category and Dairy product is under basic grocery. Dairy product is subject to the $20 limit tax exemption. Other Basic grocery has different limit.