Description

According to the comment above the class Animal, the purpose of the Animal[Symbol.hasInstance] method is to "assume that anything with the canEat property is an animal". The condition if (obj.canEat) return true; doesn't fit the purpose mentioned, because it checks whether !!obj.canEat === true. So if we create the object with falsy canEat property value (for example: let obj = { canEat: false};), then obj instanceof Animal will return false, meaning that obj is not the instance of Animal and the condition "anything with the canEat property is an animal" is not true.
I suggest replacing the if (obj.canEat) return true; condition with if ('canEat' in obj) return true;. The in operator returns true if the specified property is in the specified object or its prototype chain. With the new condition, obj instanceof Animal will return true even if the value of the canEat property is false, which is exactly the same as saying "anything with the canEat property is an animal".
P.S. There is also a hasOwnProperty() method, which returns a boolean indicating whether this object has the specified property as its own property (as opposed to inheriting it). This method is not suitable for the implementation Animal[Symbol.hasInstance], because the condition if (obj.hasOwnProperty('canEat')) will be truthy only if the property canEat is a property of the obj itself, but not of its prototypes.

Links

https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Operators/in
https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Object/hasOwnProperty